All of the OT movies were studio movies, he didn't start paying for the Star Wars movies himself til the prequels came along. Kasdan and Kershner both turned down offers to work on the prequels.
With the first movie, Lucas negotiated for and got the sequel rights from 20th Century Fox, they thought that was small beans at the time. He owned and financed ESB and ROtJ, and Fox handled the distribution for a small cut, a deal that was very favorable to Lucas. When ESB went over budget, though, Lucas couldn't get his credit extended and Fox fronted him some money to get the picture completed, in return for a bigger cut of the profits.