They've done 3 tribes in the past and they generally just merge them into 2 fairly quickly. I think it is a good system as the move from 3 to 2 sets up different dynamics in each newly formed tribe as opposed to the typical "our old tribe has the numbers in this new tribe and we will pick off the other guys one-by-one."
-Jason "CBS is committed to 2 more seasons... but this past year was the lowest rated in Survivor history" Evans