I just went through all my families (which are in their 10th & 11th generations) and this is what I found:

Singletons having twins: 8 times
Singletons having triplets: 5 times
Twins having twins: 1 time
Twins having triplets: 0 times
Triplets having twins: 2 times
Triplets having triplets: 0 times

I currently have a triplet who inherited the house but she hasn't had any children yet.
I also have 2 other sets of triplets that are not old enough yet. Looks like maybe choosing a singleton has the best chance of having triplets but I really think it's just random.