How do I make a 3 wire splice with 2/0 wire? I want to tap into a service panel feed to add a subpanel.
Are you sure this is the right thing to do? It's rather unusual. If you are feeding a subpanel from a main panel, there would be no call for doing a 3-wire splice, you put a circuit breaker in the main panel and feed the subpanel off that.
You would only use a 3-wire splice off the top of the panel in the rather unusual situation of having 2 panels of equal rank; i.e. two main panels. The new panel would never be called a "subpanel" unless the panel you're tapping off of is already a subpanel. And the panels would be right next to each other. In those cases, you'd use either screw-down splice blocks, or dual lugs.
It's difficult to infer skill from a question the size of a tweet. What concerns me is that you have the common case of adding a subpanel to a main panel, and you are freestylin' because you know that little about how this stuff works. I certainly hope not.
You use a breaker that snaps into the regular breaker spaces below the main switch, and you feed the subpanel off that. It may be quite a large breaker, which may necessitate moving even more circuits. Don't same-pole any MWBCs (and if you don't know what that means and why, you are out of your depth, and no, googling MWBC won't complete your knowledge.)