I think 5 Disney + 2 Uni is what most split trip guests are doing now, and my guess is that will still be the move for most guests when UE opens -- they'll just replace a USF day with an EU day, since USF is a stretch to even call a half day park right now. With hoppers, you could do a couple of hours at USF in the morning, see essentially everything worth seeing, then take the Hogwarts Express over to IoA for the rest of the day if you really wanted to see all three.
Once Universal makes some necessary changes to USF, then you might see people spending 3 or even 4 days at Universal, although I think it will be pretty easy to do everything you might want to do in 3 days until they have more expansions at all the parks (EU should be a full day park, but it doesn't look like a multi-day park absent wanting to ride attractions multiple times).