Help, I have a chart based on 4 inch tall cakes, but if I make 3 layers which will make each tier 5 1/2-6 inches, for a two tier cake, what size layers do I need for 40-50 servings?  I don't know how to account for the third layer when it comes to servings! Thanks!

Adding a third layer to your cake does not change the number of servings a cake can get.  All it does is make each slice a bit taller.

So base the size cakes how you normally would but make sure you adjust your price per serving to include the extra costs of that additional layer.