If I buy the AirPods 2 right now, would I be able to eventually exchange them for the AirPods 3 using in-store credit?

Apple doesn't accept exchanges for AirPods in the way you plan to go about it. You still have a few options:

1. Don't purchase the current AirPods, and just wait for when Apple releases the new ones.
2. Don't purchase the current AirPods, and just continue to your the EarPods that came with your iPhone, until the new ones come out.
3. Go ahead and purchase a current set of AirPods, and then, just buy the new model when they come out. You can either keep the current model AirPods as spares, or sell them.

doubt it, does not seem to work like that with the phones and with cars or tv's people purchase from other manufacturers