http://www.tfproject.org/tfp/showthread.php?t=89638
Kinda been answered there before.
Lots of the stuff I've been reading lately says optimum strength training occurs at 3-5 reps and 2 sets, I don't think two sets is enough, nor less than 4 reps, I just don't feel like I've done anything if I go less. So, I go three sets of four on my bigger lifts (bench, squat, ect) and three sets of 6 on machines, which I try to avoid like the plague, but sometimes you just gotta use em. I've been making consistent gains in weight and strength for about 4 months now. This is what works for me, you very probably are different. If your just doing this for your trip to cancun I doubt three weeks is going to give you any noticable difference.