transmission slipping

when I put the selector in D it goes through 1st and 2nd but when it shifts into 3rd it is like it is in neutral....If I put it in OD it will go but it slips if i try to give it gas.....

You haven't mentioned what "it" is. What year, model, whatever. People need more info to even begin to help with a problem like that. In either case it's not ikely that you're going to repair a transmission problem yourself. Assuming it's the 700-R4 (4L60), that's a very complex assembly that you're better off taking to a transmission shop for a diagnosis.