I think leaving three boxes is good.
One word of advise ... DO NOT LEAVE CLUMPING LITTER IN THE BOX ....
We went away for a week, DH was supposed to come back in a week and I was staying away for two weeks. Well DH got caught up at work and ended up staying an extra five days. I left clumping litter in the litter box, only one because it was supposed to be only five nights from me leaving and DH getting back. Well, the clumping litter turned to a conceret block and we just moved to this new place and didn't give a key to anyone down here yet. Long story short, they made our couch their new bathroom and I don't think I am going to be able to save the couch ... OUCH ...
If your cats will tolerate a regular litter you might be better off ...
Have a great trip!!! I have NEVER had a problem leaving the cats before so I am convinced it was the clumping litter that did me (and my couch) in.
ETA: The only other time we knew we were going to be gone more than 5 nights we have a pet sitter come in every other day and check on them. They aren't picky about their litter and the regular litter works well for us. I have two cats ... We just got stuck in a pickle because we had just moved a week and half before this mini trip!