Ok I know that you should have a BR of 300BB in order to play a certain Limit game to compencate for varience. So if I was playing .5/1 limit, I would need a BR of $300 right? Now what if I was mulit-tabling at .5/1 limit, say I'm playing 3 tables at once. Does that mean that I need to have a $900 BR?

I dont think playing multiple tables makes any difference. if anything it shoud smooth out your variaince allowing for a smaller roll.

Whats the difference if you play 2000 hands 1 table at a time over 6 days...or 2000 hands 3 tables at a time over 2 days?

Its still 2000 hands. The odds of going broke should be the same.