I just purchased and received my 1st Magic Line pan the 3' 10x15. My question is for those who have this pan, how many mixes should I use to make the 3" cake? I want it to be 3" or close enough because I am stacking a Cars Pan on top of it, so I want it to have depth and bulk. Any advice would be great and how far should I fill it, the same as my wilton pans 2/3 full?

If you go to the bottom of this link for Wilton they have list for 2" and 3" pans. It will tell you how many cups of batter your pan needs.

I use DH mix with an addition of one egg and one box of pudding and come up with 6 cups of batter. If you use a different brand or extender with a mix you will need to measure your batter so you know how many cups you get.

HTH

http://www.wilton.com/cake/cakeprep/baking/times/index.cfm