Tis all about perfect beat matching. With 2 decks, you only have 2 choices, speed up, or slow down. If you do one, and the beats slip out worse, then all you do is the oposite to sort it. Adding just one more deck into the situation throws this all out of the window. Make sure that your beats are perfectly matched, and then dont touch em. Ive managed to mix 6 decks, (well, 4 turntables, 1 pc, and 1 cd), with 2 mixers before, and to be honest, the hardest part is keeping track of which track is on which channel on the mixer. Other ways you can sort sloppy beats out is by nifty little tricks.

Say you have 3 tracks mixing, and some beats are slipping, but your not sure which track it is. If you kill the bass on a track, and the beats stop slipping, then its that track which is out. You can use this to your advantage by droping the bass out on the last 2 beats of a bar, and the bringing it back in,(if your quick enough then you will of sorted the slipping beats).

Another thing - Never touch the pitch on the first track which was playing. By doing this, you are only concentrating on mixing 2 decks over the top, and not trying to match all 3 together.

Hope this helps..