Generally speaking, anything less than 4 rounds feels a little incomplete to me.
But nowadays, with limited time, I'm a little more accepting of that trade-off than I used to be.
And there are other considerations: If that final round is 24 or 27 holes, it might suffice. Or if it's on a long and epic course.
One drawback to the single round on Sunday is that, if I played badly on Saturday and am way off the pace, it means possibly staying overnight, and going back the next day, for a pointless last round, since the event is already 2/3 over. In a 4-round event, the tournament's only half-over, and there's more chance for redemption.
One other thought: there used to be two tournaments in South Carolina that were 3 rounds---one on Saturday, two on Sunday.
The original Charleston Classics at Hampton Park (a terrific temp course) started Saturday afternoon, so you could drive in that morning, even from a distance, and practice the course if you wanted to. The S.C. Flying Disc Championships at Crooked Creek, at one time, started Saturday morning but then had Saturday afternoon for field events, back when that sort of thing was done, which were optional; you could participate in them, or do something else (i.e., attend a college football game). In both cases, Saturday was an appetizer, with the bulk of competition remaining for Sunday.