You don't. As redace said, they're not meant to be combined, else the wheels fail, there are a dozen copies of components, DSL2 components get mixed up with DSL3 stats and vice versa...
it's liek combining ironforge and DSL, you don't.
If you want you can import specific components from DSL2 to DSL3 or vice versa. That can be done and I think I did that with the multiextender...