How many box mixes will I need to fill a 9x3 and 12x3 pan? I'm not going to be enhancing the box mix with anything.

about 4 i'd guess

i usually use 2 for a 10x2... so im thinking 2 for the 9x3 and 3 for the 12x3??

A 9x13x2 pan uses 1 1/2
Are you talking about a 12x12x3? If so then probably 2 1/2 - maybe 3.
A 10x2 round uses 1