How do you get those? I keep getting singletons.

pure, random luck.

I hadn't gotten any until I hit 50 people, then they started coming with every 3-4 "couplings". I got 3 sets of twins and 1 set of triplets before I even hit 60 pop.

-Matt

i got em when i hit 50 too do u need lvl 3 fert. for triplets??

i have 6 sets of twiins

I just got my first set of twins - at a pop of 30/35

In my stats is stated I should have at least one triplet running around...but I never witnessed the event...and with the population on 90 it is difficult to spot them. Helas!

I've just got my first twins with a pop of around 20. I had food stocks of over 2000 after finding a crate of steaks on the beach, so my guess is that it depends on the level of food stocks.