The nice thing about a nuc for a beginner is that it is manageable. You can learn all of the basics inspecting a nuc, and they are a lot less formidable than a full hive. You don't want a massive nuc!
A national nuc should be three good frames of brood, one of stores and one that needs a bit of work. It is very difficult to sell real 5 frame nucs, because if they are full of brood, they will be ready to swarm in a week or so - an overcrowded nuc will swarm.
A commercial nuc has bigger frames, so, just like a 14x12 nuc with 3 frames of brood, is is equivalent to a standard nuc with 5 frames of brood. That's a pretty big nuc - close to a small colony in a national brood.
So my gut feel would be if a 3 frame nuc = "3 of brood, one of stores and one needing work", then that is more than big enough on commercial or 14x12 frames. If 3 frame actually means 1 brood, 2 stores and two foundation, then it is pretty small.