Google traffic is always the biggest. However that is comparing sites in the same position. If your site is #1 in msn and #60 in google you will most likely get more traffic from msn.
Most likely because of the reason I explained above - your yahoo ranking is higher than your google ranking.
I don't know about others but for me, the msn traffic is almost negligible. In other words, it doesn't matter whether it's there or not