how do you get triplets?

I've managed to get twins buy watching lots of kids tv and having my sim read kids books. How do you get triplets? I haven't managed that yet? Is there a sure way to do this? I have heard and read that there is a pregnancy mod to download but haven't found it yet. I would prefer to try getting them without a mod if there is a trick or potion or cheat in the game that I don't know about to get them.