Do I need a heating core with a 10x10x3 pan? Thanks

I would, I use the flower nail as a heating core.

I also use the flower nail. LOVE IT! Will never use that big 'ol heating core again. I use the nail for anything above a 8" or 1/4 sheet. But I also have a tempermental oven.

thank you so much for the help

of subject but do u just put the flower nail upside down in the center of the pan and pour in the batter?

Yes. Spray it real good with pam and put it in upside down in the center of your pan.