Centinje was probably the only disappointment I experienced in Montenegro.
As for 3 nights in Kotor. If you have seen or will see Dubrovnik on this trip, then spending even one night in Kotor isnt well spent. Its like Dubrovnik, but not as good.
If I had four nights to spend in Montenegro again, I would do a night in Perast a perfect lay over after leaving the crowds of Dubrovnik. Beautiful setting on the Bay of Kotor. A night i Budva. Just a fun, not overly crowded seaside walled town. We did a little deep sea fishing out of Budva. Ulcinj for east meets west. The old town on the hill is really, really amazing. Then I would cap it off with a night in Mojkovac in the Montenegro high country. Acutally, I would do that 3 nights. Of course I would be there for the canyon nearby, the nature and the fly fishing...
Montenegro is one of my favorite places. But you are missing everything I love about it.
Still, eveyone's interests are different. I bet no matter what you come up with you will have a wonderful time.