I am no mathy guy either, but from my limited understanding of statistics, bigger sample size means more convergence towards the mean.
If you are a winning player (and making the rather simplistic assumption that your win rate is unaffected by 3 tabling as opposed to 1 tabling) this means the more hands you play in a given time period the more you are likely to win and the more your good and bad luck will balance out (as variance due to luck will presumably converge to zero over enough hands, unless of course you are Chris 'God Bless Chris' Moneymaker

So by 3-tabling you should increase your actual winnings threefold with and achieve lower variance per hand played (sure you play more hands and if you are playign badly you can flush money away real fast, but if you are playign well enough to have a +ve expectation then it really must be the best thing to do in the long run)