I don't have a lot to say about these TPB's. X-Men sell, the 1989 What If? series was very X-heavy heavy, with X-stories every other issue for months, towards the end. For the Volume 1 cover, I'd use issue #23 - a homage to Giant-Size X-Men #1. For the Volume 2 cover, I'd use issue #84, just because it has multiple X-Men on the cover. For Volume 3, I don't really like any of the covers as representing the team, but I'd use issue #110, just because it has Professor X, Phoenix, and Colossus on the cover - characters I think of as core X-Men.
Would you buy it? Let me know in the comments, and thanks for reading.