As someone who has done 2) 3-night cruises, 1) 5-night cruise, and 1) 7-night cruise; I would say the 4 or 5-night cruise on the Fantasy. I make that recommendation for a couple of reasons:
1a - Like others have said, 3-nights is very short for a first time cruiser. My MIL came with us for her very first cruise on a 3-night Wish cruise 2 years ago and by the end of the last day, she was upset that it was over already and wanted to stay on board longer. This is coming from someone who was deathly afraid to even go in the first place, but went because our kiddos (her grandkids) 'talked' her into it. It was a massive surprise to the kids at the airport because they didn't know she was coming.
1b - DW and I took our first
DCL cruise as adults only on a 3-night sailing on the Dream with another couple. We had such a great time, we were sad when it was over 2.5 days later and were already having to pack up our suitcase and putting it outside our cabin to get ready to debark the next morning.
2 - IMHO, I feel like the Dream class ships (I've sailed the Dream, you're looking at the Fantasy) are much more 'user friendly' than the Wish. I'm NOT saying the Wish is a bad ship, just that having the walking track, the flow of the ship, and the more distinctly separated adult areas make for a smoother sailing experience.
3 - Our family has another 5-night cruise right around the corner because we want to enjoy all DCL has to offer, make the most of our available days, get to know our wait staff at dinner, and get to the point of feeling at home.
4 - ALL THAT TO SAY THIS: You're choosing to cruise for the first time on DCL. Don't obsess too much about the minutia. Be excited you're going on a cruise and enjoy the experience, regardless of which cruise you ultimately decide. Coming from someone who didn't think they would enjoy Disney as cruise line, I thoroughly enjoy what DCL has to offer. I try to make the most of the trip each time I'm on board.