With the LDS view that the Trinity is three beings would that mean they (LDS) are monotheistic or polytheistic?
There is more to polytheism than simply believing in multiple divine beings. One of the hallmarks of polytheism is multiple gods, each with their own domain and their own agenda.
To use a culturally familiar example, in the Greek pantheon, Zeus was the father of the gods, god of the sky and thunder and lightning. Hera was his consort, and she was constantly squabbling with him because he was constantly cheating on her with mortals. From one such indiscretion sprang a great hero who Zeus named Herakles (aka Hercules), meaning the glory of Hera, to try to appease her wrath. (It didn't work. She hated him.) Meanwhile, they had Ares (God of War,) Hades (God of the Dead,) Poseidon (God of the Oceans,) Aphrodite (Goddess of Love and Beauty,) Athena (Goddess of Wisdom,) and several others. And they were constantly fighting with each other, either directly or by proxy through meddling in the affairs of mortals. Similar social interactions are seen in polytheistic systems all over the world, throughout history.
By contrast, Latter-Day Saints believe that the Father, the Son and the Holy Ghost are each individual divine beings, each with different roles in administering God's plan, but their "domain" is all of creation, and they act in perfect harmony and agreement. Together, they are our (single) God, in much the same way as various forms of governing councils work and act together here on Earth. (And the more harmonious such governing bodies are, the more efficiently their agenda tends to be accomplished.)
The fact that we say that each of the three is a God (a divine being) and that together, they are one God (the ruling body of Creation) is more a failing of the language than a bad description, much like the way a man could say "I love my wife" and "I love chocolate" and mean two different things by the word "love".
They consider themselves monotheistic. They do not consider Jesus, or the Holy Spirit to be God. Only God the Father is God, in LDS doctrine, and they worship only Him, recognizing the other two to be His agents.
To acknowledge the scriptural evidence that otherwise perfectly united members of the Godhead are nevertheless separate and distinct beings is not to be guilty of polytheism; it is, rather, part of the great revelation Jesus came to deliver concerning the nature of divine beings.
The standard position I hear often in conversation with members of the LDS church (but it may just be the folks I associate with) is that Mormonism is either monotheistic or henotheistic/monolatrous. Monotheistic because of the same reasons that Mason noted above, and because God the Father is The God of the Universe and is worshipped by Jesus Christ as well. Henotheistic/monolatrous because we admit the possibility of becoming joint-heirs with Jesus Christ, and thus gods, but no one besides Jesus Christ is to be worshipped in that sense---and he's really a special case because he was divine before the world was.
In any case, I doubt that you'd offend a Mormon by calling them casually tritheistic in conversation.
On second thought, the BYU-crowd conceit of using the term henotheistic may be inaccurate, since it implies not only worship of one god but that worship of other gods by other people may be admissible. So monolatry is probably a better term for it. But you can definitely find people who took their religion professors at BYU very seriously who will raise the issue.
Mormonism creates definitions for polytheism and monotheism that deviate from traditional or standard lexigraphic uses. A mere belief in multiple gods, according to traditional and lexigraphic sources, is indeed sufficient for polytheism.
Another responder here stated that Mormons don't believe Jesus is God - and that only the Father is God. That is not correct. They are both (G/g)ods to Mormons. For that matter, even the Mormon belief that "God the Father" is "God of the Cosmos" isn't quite the whole story; Mormons believe that God the Father was a mere man who became divine by the power of a yet another God who existed before him.
But it is true that Mormons, with their peculiar definition of "monotheism", consider themselves to be "monotheistic." This is best understood by looking at a careful explanation from one of their own prophets:
Journal of Discourses 1:56 - p.57, Orson Pratt, August 29, 1852
All these beings of course are one, the same as the Father and the Son are one. The Son is called God, and so is the Father, and in some places the Holy Ghost is called God. They are one in power, in wisdom, in knowledge, and in the inheritance of celestial glory; they are one in their works; they possess all things, and all things are subject to them; they act in unison; and if one has power to become the Father of spirits, so has another; if one God can propagate his species, and raise up spirits after his own image and likeness, and call them his sons and daughters, so can all other Gods that become like him, do the same thing; consequently, there will be many Fathers, and there will be many families, and many sons and daughters; and they will be the children of those glorified, celestial beings that are counted worthy to be Gods.
Journal of Discourses 2:345, Orson Pratt, February 18, 1855
"But" inquires one, "how are you going to get along with the passage, in Isaiah, where the Lord declared that, 'There is no God before me, nor shall there be any after me?'" How can we believe this, when we believe in the revelation given through Joseph Smith, which says there are many Gods, and that Abraham, Isaac, and Jacob are Gods, and that all good men in this Church shall become Gods? Paul also speaks of the only wise God. Perhaps some may suppose that it is translated improperly. But you will find the same thing in the Book of Mormon, translated by the Urim and Thummim; the same things are also contained in the new translation of the book of Genesis, given to Moses, where the Lord declares that, "There is no God besides me." In these expressions, God has reference to the great principles of light and truth, or knowledge, and not to the tabernacles in which this knowledge may dwell; the tabernacles are many and without number, but the truth or knowledge which is often personified and called God, is one, being the same in all; God is one, being a unity, when represented by light, truth, wisdom, or knowledge; but when reference is made to the temples in which this knowledge dwells, the number of Gods is infinite.
Journal of Discourses 2:345 - p.346, Orson Pratt, February 18, 1855
One world has a personal God or Father, and the inhabitants thereof worship the attributes of that God, another world has another, and they worship His attributes, and besides Him there is no other; and when they worship Him they are at the same time worshipping the same attributes that dwell in all the personal Gods who fill immensity. And hence the Lord says, in one of the revelations of these last days: "Ye are tabernacles in which God dwells, man is the tabernacle of God."
In short, because each God of each world shares attributes with all Gods of other worlds, these shared perfect attributes (light, truth, wisdom, knowledge) are the embodiment of the Mormon-peculiar definition of "monotheism."
However, true monotheism requires much more than mere shared perfect attributes. Indeed, the notion of "sharing" is problematic itself. True monotheism poses God as "first cause". Mormons deny that their "God the Father" is "first cause." And yes of course, it also includes the notion that there are no other personal-being gods existing anywhere. For true monotheists, of a Christian slant, the bible's reference to men as "gods" is a metaphor, not an assertion of multiple divine beings.