I usually make 2-layer cakes, but I have someone wanting a 3-layer 10" cake. Does anybody know how many servings that's considered to be?

I have a 2 layer 10" cake serves 38 ppl, so add another layer on there (19 servings) so I would say 57 servings.

If it's a party, I'd quote the wedding serving of 38, and the same for a wedding, because you really don't want to cut a smaller serving than a 1x2in, The other way to do it would be to make 2 x 3in deep cakes, cut those into 2 layers and fill, but dowel and put a cardboard in the middle of the two cakes, so in effect you had 2 10x3in cakes, each one serving around 24, but I would only do that with a fondant cake, because you need to top the bottom cake with a circle of fondant before topping with the other cake (on a board), and then covereing the whole cake in fondant.