One thing you can use 3 GCs, but I've heard of CM give people a hard time sometimes about it. Because to do more the 2 (or 3 maybe) forms of payment apparently they have move the 3 GCs to a "any value" card for the total of the 3, then use the Any Value card to buy the tickets.
If you are staying on-site is MIGHT be easier to put it as a room charge then apply the GCs to your room. Same end effect at the end of the day.
If the person at the window says it can't be done, ask for the manager. It can be done, it's just complicated.