It's pretty common to find cake recipes with pans that don't match the size of pans you have. I know ideally you would make sure total area of the pans is the same as in the original recipe, so that the batter is the same depth and it bakes properly. For example, two 10-inch pans are about the same area as three 8-inch pans.
But if nothing works out that cleanly, and I don't want to just scale the recipe, how much thicker or thinner can you make a typical cake without really messing things up? For example, using two 9-inch pans instead of three 8-inch pans would make it about 20% thicker - would that be likely to cause problems? What about the other way around, 20% thinner?
(I know the baking time can change, but it's easy enough to check for doneness.)