Bendis addressed this in an interview, and said he'd be getting back to Jon's time on Earth-3 in Legion through flashbacks. But even outside of that, there are several statements and clues that make it clear that he likely spent the majority of his time there outside. Not only does issue 9 of Superman have Jon specifically say that he traveled around Earth-3, but it also says that he helped and interacted with people who sometimes didn't always receive his help well. Then over in issue 16 of Superman Jon tells Damian that the food on Earth-3 is better than Earth-0's because of a lack of artificial flavoring, so clearly he was out and about.