I have to make a cake in the morning to be delivered in the evening tomorrow. Can someone please tell me more or less how many cake boxes would I need to use to make a WASC in a 17 x 12 pan 3 inches deep & will it cook evenly this is the first time I'm using this pan it's to make a shirt cake.

A full recipe of Rebecca Sutterby's WASC using 2 DH white cake mixes will yield a tad over 14 cups of batter:

http://tinyurl.com/2cu8s4

In order to determine how much batter you'll need, fill a one cup measure with water and fill your pan 1/2 full. (However much water you used, this is the amount of batter you need.)

Use bake even strips and inverted flower nails, baking at 325F until cake test done.

For approximate baking time find the closest sized pan on these Wilton charts:

http://www.wilton.com/cakes/making-cakes/baking-party-cake-3-inch-pans.cfm

FYI, a 12x18x3 pan requires 20 cups of batter and bakes for 85-90 minutes at 325F.

HTH