Well you have to ask yourself a couple questions...
Is that many sets how many sets you've been growing off of? If not, you need to switch it up, either lower your intensity and raise volume or raise intensity and lower volume (rep ranges work wonders too).
Can you get the routine done in under an hour? Because by going to a 3 day split you're combining some of the routine into the same days and making your workouts potentially longer. Unless you're cutting out sets or something of course. If you cant get it done in under an hour, you need to raise your split to a higher number of days.
4 day or 5 day splits dont instantly mean overtraining as a lot of HIT-nazi's like to believe. As long as you arent overworking a muscle during a week and it has ample time to recover, you're fine. (this is very different for everyone though, we all respond to different stimulae, and overworking a muscle can have completely different criteria from one person to the next depending on recovery ability, etc)