I use 3 of the ateco heating cores in my 12inch round cake pans, should I use 4 (one in each corner for the 12x12x2 square cake pan?

For what it's worth, I've never used a heating core or flower nail either. I've made quite a few 16" round and 10" square. I just lower my temp to 300.

^ agree.  My experience is that heating cores and flower nails are nonsense.  lower the oven temp and bake on!

I used to use flower nails.  Now I just bake at 325 and use Magic Line pans.  I have done this all the way up to 14" pans with perfect results.

I totally agree w/ jgifford and leahs.  

No baking nails or heating core found in my kitchen. I've baked thousands of cakes and never had an issue. 

I use to never use heating cores... however, I recently started using the Ateco cores when I use poundcake recipes for wedding cakes... I already bake them slow (but often the edges cook before the middle..  With the Ateco cores... they cook quicker an more even (still on low temp).  The edges and corners are just as moist and soft as the middle... Love them!  Oh... I used 4 in my 12x12x2 square pan and it cooked even better! And they're pretty cheap!