Would I need to use a heating core or 2 or more flower nails for a 12x18x3
pan? Thanks

I use 7-8 flower nails in my 12x18x2 pan. Since yours is 3" high I would use even more.

And just to illustrate how things work differently for different folks .... I never use nails or heating cores for any of my cakes, not even the 14x22.

I use nails only, not cores. I bake from scratch and find it works best for my cakes, but your mileage may vary!