The question comes down to whether Paul, Mark and John failed to mention the virgin birth because:
- They never heard of it or
- They did not think it was necessary to include in their writings.
Each of these are different cases, so I will take them one by one.
Galatians, as John McKenzie mentioned, is as good a place for Paul to have mentioned the virgin birth as any. He explained his purpose for writing early in the letter:
I am astonished that you are so quickly deserting him who called you in the grace of Christ and are turning to a different gospel—not that there is another one, but there are some who trouble you and want to distort the gospel of Christ. But even if we or an angel from heaven should preach to you a gospel contrary to the one we preached to you, let him be accursed.—Galatians 1:6-8 (ESV)
We know from the Talmud that Jesus' parentage was a common line of criticism of Christian gospels. So one might very well assume that since Paul didn't defend Mary's virginity, he didn't believe it or didn't think it an important part of the gospel. However, Paul's letters predated any of the works we call "the Gospels". In Galatians, it might be better to translate εὐαγγέλιον literally as "good news". Specifically, Paul is talking about the good news of the "grace of Christ" and spends nearly the entire letter fleshing out the reason Jewish Christians are no longer held to the Mosaic law.
We don't know exactly how the good news was originally communicated to the Galatians (and other early Christian communities), but the written gospels show clear evidence they began as oral traditions. This isn't surprising given the importance of the oral Torah at the time. As a rabbi, Jesus was expected to teach on the law and his disciples were expected to remember both the teachings and the context of individual lessons. As with the two major schools of Jewish tradition, followers of Jesus would have taught their own disciples the teachings of their rabbi. Of particular importance to the question, the circumstances Jesus' birth would not have been part of the oral tradition since Jesus didn't take that opportunity to teach (understandably).
So one possibility is that Paul didn't include the virgin birth information because it wasn't part of Jesus' oral tradition. But that doesn't mean the virgin birth wasn't taught in other contexts. Luke 1:1-4 mentions specifically "eyewitnesses and ministers of the word" (ESV) as sources. It's entirely possible Paul didn't know about the virgin birth because he was more concerned with the oral tradition than with eyewitness accounts of Jesus' early life.
Regarded by most scholars as the earliest gospel account, Mark seems to follow a similar approach to Jesus as his contemporary Plutarch did in his Lives of the Noble Greeks and Romans. In particular, Mark's gospel attempts to paint a portrait of Jesus through his actions. Like many of Plutarch's Lives, the first gospel drops us in medias res with a description of John's baptism of Jesus. His mother and brothers are mentioned briefly, but mostly serve as a motivation for a teaching about community.
The core message of this gospel can be found in Mark 8 when Peter identifies Jesus as the messiah. Then Jesus subverts expectations by predicting his suffering and death rather than a triumphant victory over the Romans. The author then tells of how this prediction came true. Importantly, Jesus also predicted his own resurrection, which is not described in the gospel. It's possible this section of manuscript was lost, but it could also be the author's intent. If so, it's not really surprising the virgin birth narrative was skipped too.
Like Mark, John skips the nativity. Matthew and Luke added nativity stories to the start of their accounts, but otherwise accept the Marcan chronology. As (most likely) the final gospel, John's author would (probably) have been aware of the those narratives. But he uses his own chronology and borrows almost nothing from the synoptic tradition. Instead, he begins with a philosophical essay equating of Jesus to the concept of Logos reminiscent of Philo of Alexandria.
Like Paul, John had an opportune moment to mention the virgin birth in the Bread of Life discourse:
So the Jews grumbled about him, because he said, “I am the bread that came down from heaven.” They said, “Is not this Jesus, the son of Joseph, whose father and mother we know? How does he now say, ‘I have come down from heaven’?”—John 6:41-42 (ESV)
Like in Galatians, this is a fine opportunity for the author to point out Joseph was not the biological father. (The author parenthetically points out later in the chapter that Jesus was talking about Judas.) Also like Galatians, the virgin birth is not critical to the purpose of the text:
The true light, which gives light to everyone, was coming into the world. He was in the world, and the world was made through him, yet the world did not know him. He came to his own, and his own people did not receive him. But to all who did receive him, who believed in his name, he gave the right to become children of God, who were born, not of blood nor of the will of the flesh nor of the will of man, but of God.—John 1:9-13 (ESV)
Notice that this passage asserts that not just Jesus but all who believe in his name haee the right to be born of God. While the virgin birth is unique in history, John asserts that spiritual rebirth is available to all who recognize Jesus' true nature. So discussing the details of the nativity would have been a distraction from the main thesis.
This is certainly an argument from silence that hardly seems justified. The virgin birth doctrine must have been ridiculed early and would have been a distraction for Paul and John. Mark's biography probably left out Jesus' parentage for stylistic reasons. Matthew and especially Luke add details of Jesus' birth to Mark because they were interested in preserving those traditions.
As an aside, it's easier to see that the argument from silence is tenuous when you read modern documents with a critical eye. For instance there's an article in the LA Times this morning today about the possiblity of race riots. It's a complicated issue with many contributing factors. While the article does highlight some new factors, it assumes people know or can easily discover details that were relevant 25 years ago. There's no doubt the acquittal of Rodney King's attackers was a pivotal event, but it's not mentioned in the article at all. It's just part of the background knowledge the reporter assumes everyone will have. I suspect Paul, Mark and John felt the same way about the virgin birth.