How do I copy Schedule 3

What do you mean by copy? Are you trying to claim the same amounts on your spouses tax return? 

I know there is a way to split Capital gains and losses with a spouse by copying.  I just can remember how to do it.

Which product are you using? the online product or the desktop installed product?

desktop installed.

If you go to the Forms mode, then when you are on the Sch 3, go to EDIT in the menu bar, then click on Forms, then click on Copy/Transfer. Just be sure you are not double reporting anything...if you are splitting gains (like 50% each), make sure you only enter 50% on each persons return.