i am making a 10" round and 12" round both 3" high. if a heating core is needed is there anything that I can use instead of going out and buying one?

I have never used a heating core for any of my big cakes I just make sure that I bake them longer and keep testing the centres.

if you have a flower nail, you can use that as a heating core.

grease it up, put in center of pan, standing on its head.

as some here have suggested for really big pans, can even use multiple flower nails

I used the flower for the first time a couple of days ago and now I will always use one. It really works and I didnt get a dome like I usually do.

That flower nail idea is great! I am going to try that next time!

I have even started using the flower nail in my 8in cakes too. Works great. For a 12in I use 2 (I'm a bit overzealous but I don't want to deal with an undercooked cake!)