Honestly, I’d try using everything at once. If you have the Spa Day pack, you can go to the spa and get a “Fertility Massage” or whatever it’s called. It will make your sim more fertile for 12 hours whilst also increasing the chances for triplets. I’m sure there’s probably a whole lot of other things through other packs that may help, too! I’ve had one of my sims have triplets by surprise, but it was just from using the Fertile trait. I hope this helps in some way!