He just wasn't on set all the time. Otherwise he was just as involved in creative process as he was in other two films.
Right. Lucas was pretty involved with "Empire"- involved enough that he wrote "Empire Strikes Back" draft 2 and 3 (which includes most of our favorite scenes), hired both Leigh Brackett for Draft 1 and Lawrence Kasdan for Draft 4.
He was on the phone with Irvin Kershner during shooting and showed up every once in awhile. In fact both him and Kershner had identical flip books with the entire movie's storyboards so that they could discuss things more easily on the phone.
He was also involved enough to completely edit "Empire" himself- the problem was that it sucked (if I recall correctly)