Is there a magic way to not get a cake to sink the middle with a 14 inch pan?
I used a mix, and maybe I used too much batter?
A 14" round 3" deep cake pan requires 15 cups of batter.
Link to Wilton's cake preparation charts:
The 3" pans can be difficult to work with especially when using scratch recipes because the amount of leavening (when making multiple batter recipes) becomes an issue.
Did you doctor the mixes at all?
I've had success with the expanded flavors version of WASC cake:
(I always use DH cake mixes.)
Using multiple inverted flour nails, bake-even strips and baking at a lower temperature all help.
Cake trouble shooting charts: