What hose do I use to connect my GE dishwasher to my sink's 3/4" Tee?

You'll want a barbed connector similar to this one:

^{Image courtesy of Lowes.com. Note that's not the right size, nor is it an adapter, it's just for illustration purposes.
No endorsement intended or implied.}

You'll want it to have a 3/4" barb on one end and 7/8" on the other end. You can use either plastic or brass, it doesn't matter.

I'd also suggest that you pick up hose clamps to put around where the hose ends go over the barbs, just for a bit of added security. A 1" hose clamp will adjust small enough to cover both the 3/4" and 7/8" connection with no issue, so a pair of them will be just fine. You'll probably want to be sure you've got one to attach the extender to the disposal intake as well. Better to spend an extra $2 (for expensive, fancy clamps) and not need them than to clean dirty, soapy water out from under the kitchen sink.