Triplet Trouble

I've been playing Sims for five years now on the PlayStation and not once have I gotten triplets. It's quite upsetting as I really want them. I've tried everything to from give both parents the fertile trait and now I have spa day and even that's not working. I don't know if it's the same for other people but it seems the algorithm is making it to hard to get triplets. Does anyone else have this problem.