I have and oven with 10/3 mc cable attached. I am going to splice this to a 10/3 romex cable that I pulled from the sub panel. Is a metal 3" deep single gang box big enough? How many cubic inches do I need?

Thank you

Need to allow 2.5 cubic in for each insulated #10 wire, and 1-2.5CI for all grounds.
6- #10 x 2.5 CI= 15 + 2.5 for the grounds=17.5 cubic inch minimum

A 4"x4"x3" box has 48 CI of space ,so the box is fine

tkrussell,
Again, many thanks for your help.