I did not do a 3 night until I had done MANY longer cruises on
DCL. At that time we did a 3/4 B2B.
There is a huge difference between a 3 and a 4 night cruise. The biggest difference is in the attitude of the guests. On a 3 night, they are crazy! Many are convinced that they have to do "everything" in 2 1/2 days on the ship. This is, of course, impossible, but they will go nuts trying!
OP had it right--choose a few things that you want to do and concentrate on those. Book them ahead if at all possible. And stay out of the way of the crazies!
The other big thing we experienced was lack of supervision of the kids on the 3 night. It was like the parents decided that their children were DCL's responsibility and the parent was on vacation...end of discussion. We had a large extended family across the hall from us. THe kids would bang on OUR door looking for their cousins (didn't they know that the cousins didn't have fancy magnets on their doors??? and where were the parents while the kids ran the halls screaming?) There was a 6 year old in the Quiet Cove pool--ultimately removed by a CM after many guest complaints. Actually there were kids all over the adult areas.
You can have fun on a 3 night cruise. However, having seen the difference between a 3 and a 4 night, I'd opt for at least a 4 if possible.