There's very little, bordering on nothing, you can do in 3 weeks.

You already know you need to swim more. So do. Not for this race, but for future races. Swimming takes time. Many months, and years. If you aren't a fish, you will need to swim 5-6 times a week for an hour each.

How much are you running per week?

Why not join a tri club? They will have a coach and a training program to follow. You will likely improve much more and faster than you would on your own.

Edit: you could practice transitions. That's something you can do in 3 weeks. You can also put in some OWS swimming with a wetsuit to get used to it.

I've been running about 2 hours per week, usually two 30-40 minute runs and one 60-70 minute run.

I understand that from a fitness standpoint, there's not much to be done, I just want to make sure I don't do anything to hinder my performance.