1.) If the player uses Gelnon's ACTION text, does he also get the rerolls from the Romulan Flagship card (if he is within the proper range of it)?
Yes. Gelnon's action counts as an attack and the Romulan flagship is a continuous effect, so would be usable.
2.) Would he also get the +1 attack dice from Donatra's ability as well (if he is within proper range of her ship)?
Yes, as above.
3.) If both of the above are true, then the player would get to roll 3 attack dice and get the re-roll from the flagship on this action/attack, correct?
Some links here: http://iraw.wikia.com/wiki/Gelnon