The apostle John uses the word world (Greek transliteration, cosmos) 26 times in 1 John, 2 John, and Revelation (NIV). Here's how he unpacks the word in 1 John 2:16,17:
"For all that is in the world, the lust of the flesh and the lust of the eyes and the boastful pride of life, is not from the Father, but is from the world. The world is passing away, and also its lusts; but the one who does the will of God lives forever."
Since your triumvirate includes the world, the flesh, and the devil, there is some obvious overlap with John's unpacking of the word cosmos. It seems to me that your word flesh subsumes the two lusts mentioned by John (viz., the lusts of the flesh and eyes).
The word flesh (Greek transliteration sarx) in the NT denotes what I call the anti-God principle (or tendency) which resides in the hearts of all of us. Since the fall of the human race, which occurred when our first parents sinned by disobeying God, every child of Adam's seed has this bent toward sin. Parents need not teach their children how to sin; it comes natural to them.
Our physical appetites (e.g., hunger, thirst, sleep, and sensual-and sexual pleasure), though God-given and God sanctioned, constitute one instrument of the flesh, but only when we attempt to satisfy those appetites in ways which God forbids. Whatever God forbids, He does so for good reason. Simply put, God says "Thou shalt not" because He loves us, He wants what is best for us, and He wants to protect us from unpleasant and often unanticipated consequences. In that way, God is very much like a good parent. It is no accident, of course, that one of the many names of God is Father.
Elsewhere in his first letter, John tells us,
"We know that we are of God, and that the whole world lies in the power of the evil one" (1 John 5:19 NAS).
In this verse we learn that the devil (i.e., satan, the evil one, Beelzebub--see 2 Kings 1:2-3, 6, 16) exercises his power in and through the world and its system. The world in that sense denotes the fiendishly organized effort of satan and his minions (also known as "rulers, powers, world forces of darkness, and spiritual forces of wickedness in the heavenlies," 6:12) to "kill, steal, and destroy" (John 10:10) two things in particular:
everything God declared to be "good" from the very beginning, particularly man and woman whom He created in His own image
the plans and purposes of God, particularly as they relate to the unfolding drama of salvation which will ultimately triumph over satan, when
"'The kingdom of the world has become the kingdom of our Lord and of His Christ; and He will reign forever and ever" (Revelation 11:15 NAS).
Until that glorious day, Christians must confront the same enemies our first parents, particularly Eve, confronted:
The lust of the flesh: "the woman saw that the tree was good for food" (Genesis 3:6)
The lust of the eyes: "the woman saw that the tree was . . . a delight to the eyes" (ibid.)
The pride of life: "the woman saw that the tree . . . was desirable to make one wise, [so] she took from its fruit and ate; and she gave also to her husband with her, and he ate" (ibid.).
Jesus came to this sin-cursed world, John tells us, to defeat Satan once and for all.
"for the devil has sinned from the beginning. The Son of God appeared for this purpose, to destroy the works of the devil" (1 John 3:8 NAS).
Our Lord's death on the cross dealt Satan a death blow, as it were (see Genesis 3:15), and Paul tells us that in the wake of Christ's victorious cross death, our Lord bound Satan and paraded him throughout the universe, giving him a taste of the shame, punishment, and torment that will be his to bear when one day he will be cast into the lake of fire forever and ever (Revelation 20:10):
"Therefore it says, "WHEN HE ASCENDED ON HIGH, HE LED CAPTIVE A HOST OF CAPTIVES, AND HE GAVE GIFTS TO MEN." (Now this expression, "He ascended," what does it mean except that He also had descended into the lower parts of the earth? He who descended is Himself also He who ascended far above all the heavens, so that He might fill all things)" (Ephesians 4:8 NAS).
In conclusion, the Christian's enemies are certainly the world, the flesh, and the devil, though as I've pointed out above, we can, without doing violence to the text of Scripture, expand them to include the notion of lust, which goes hand in hand with the flesh.
Are there other enemies of which Christians should be aware? Yes, and I think that Paul's description of the armor of God in Ephesians, chapter 6, would have to include doubt, discouragement, and defeat, particularly when we fail to put on "the full armor of God" (6:11).