What's average for height of a tier? I measured my last cake (chocolate) and it was just above 3 in. But my scratch yellow cake was not quite as high and feel like I. Will need to bake an extra layer for it.
4" or two 2" layers.
US is 4"+ (Averages between 4 and 4.5" depending on filling, layer height, coating and whether you are using fondant)
UK is 3"+ (Averages between 3 and 3.5" depending on filling, layer height, coating and whether you are using fondant)
Don't worry about whether your cakes are average. Just decide what look you are after and decide from there what you need to do. Cakes are so individual and the looks we are trying to achieve are individual.
If it needs to be the same height as your 3" cake, you could do one of two things: #1. how much shorter is this cake that you are talking about then your 3"? is it enough that you could bake another real thin layer and just put enough icing or filling between what you have and the thin layer to stick together or if you were to torte the slightly shorter one and put icing or filling in-between and icing it all over to make it equal to the 3" in height?
Most people that choose to have someone besides a bakery make a cake for them is for several reasons. They can personalize it more, it tastes better, and usually doesn't cost as much because the person doesn't have the overhead that the business does and can afford to not charge as much.
I don't know that you are doing a cake for someone else, but I just thought that you might be. I hope my suggestions help.