I need 3 credits in the summer to graduate h.s. How can I earn these last credits?

Normally, you would do this at the high school you went to during the school year. Is that not possible for some reason? In that case, I would ask them about how to do this. I presume you would get your diploma from that school, rather than from some place you would go just to get three credits, and so you need to be sure that whatever you do meets their standards.