inch tall cake.

Hpw do I do this without baking 2 cakes and leveling?

Add height to your pan by making a collar with foil or parchment paper. Put it around the inside of the pan and spray/prepare the pan as you normally do. I do this for most cakes I make.
I can't tell you how much batter you need to make a 3-inch cake, however.

Thanks so much, I'll have time tomorrow evening to play with batter.

there is an article on how to collar your pan, and i used wax paper, hope this help, if you can't find it another good tip is to use the cake batter like a glue along the sides of the pan

here is an article on CC to collar your pan:

http://www.cakecentral.com/article43-Collaring-Your-Cake-Pans-Make-A-Deeper-Cake.html

I'll be doing a collar on a 11x15 also this week for the same reason as you. I've never collared anything other than a round so I plan on trying to make a small crease where the corners will to try to hold it in place better while pouring in the batter.