I bought a new alternator and it bolts up to my 83 toyota pickup, but the connector plug of the new alternator is 2-pronged, and the original plug is 3-pronged. What would be a solution? The auto store took the old alternator already 3 days ago.

 

They gave you the wrong alt. take it back and get the right one.
Maybe have a pic on your phone of the plug end that you have so you can make sure before you walk out.
I know that there were some changes made around this vintage for the alt wiring, but don't remember the specifics. I'm pretty sure there's a thread on here somewhere.

 

I think you’re exactly correct. I uninstalled it and got the refund while an alternator is coming from Summit.