Yeah it's definetly really random. I had a triplet (my only one so far), before I had twins. I've completed all the puzzles and collectibles and techs, but haven't been able to get more triplets. I had that first set of triplets on a population of about 30, but only a few twins since.
The only thing I think I have figured out is that it directly links into who the father is.. Certain villagers seem a lot more fertile than others...