Gambt is one of those characters that makes the short list, but is replaced by more interesting people. X1 only had a few characters, it was about Wolverine and Rogue joining. X2 centred on Nightcrawler as a new mutant. X3 included Beast and Angel as new characters, and so they decided when Gambit is introduced, he'll be a main character.
As for Jubilee, well she has been in all three films. Rogue and later Shadowcat (Kitty) seem to take the place of the 'female teen X-Man', so Jubilee is left at the side.
In the first one you can see her with her trademark yellow mac in the classes with Kitty and Rogue.
In the second one she has her own deleted scene where she nearly uses her powers in the museum. She is later one of the six students to be kidnapped. Storm calls out for Jubilee and she replies. She also appears in one of Professor Xaviers classes at the end, and asks if he's alright.
As for the third one, by rights she should have made the team, but once again was ignored. She can be seen in yet another classroom scene (she sits in the middle of the room, next to Kitty), and briefly during the funeral.
Kea Wong has played her for the past 2 films. Hear's hoping she'll be an X-Man by X4!