Unbelievers sometimes say they reject the virgin birth because it's only attested in two Gospels.
i) Since unbelievers typically reject miracles a priori–since, indeed, unbelievers regard an account containing miracles as automatically discrediting the historical reliability of the account–this objection is duplicitous.
ii) We wouldn't expect the virgin birth to be attested outside the Gospels. The NT letters aren't histories or biographies. They contain only occasional references to the life of Christ. Same thing with Revelation. Acts is a history of the establishment of the NT church.
iii) Because John's Gospel is generally considered to be the latest Gospel and the most theologically "advanced," unbelievers regard it as the least historical. If, therefore, it reported the virgin birth, they'd discount that in the same way they discount John's high Christology, the miracles of Christ in his Gospel, as well as the speeches and dialogues of Christ in his Gospel.
Indeed, Andrew Lincoln, who's penned a critique of the virgin birth, also penned a commentary on John's Gospel, and he doesn't put much stock in the historicity of John's Gospel. So, for critics like him, it wouldn't matter if John recorded the virgin birth.
iv) Finally, if all three Synoptic gospels attested the virgin birth, unbelievers would regard that as even less impressive than if only Matthew and Luke attest the virgin birth.
Assuming Markan priority, if all three Synoptic Gospels attested the virgin birth, unbelievers would discount the testimony of Matthew and Luke because they'd say Matthew and Luke simply copied that from Mark. Rather than multiple-attestation, they'd say that boils down to just one Gospel.
Conversely, since Mark doesn't record the virgin birth, that means the witness of Matthew and Luke does constitute independent corroboration. Since, in this case, Mark is not the lynchpin connecting Matthew and Luke (vis-à-vis the virgin birth), they didn't get that information from Mark, or from each other.
So we have two Gospel authors, writing independently of each other, bearing historical witness to the virgin birth.