I noticed this also on PS4. I did the 26 generation legacy achievement and throughout that whole time, I had each sim purchase fertile trait from the rewards store. No twins or triplets ever. I then was able to use fertility massages and fertility drinks (OR) and the same thing. Single births even using the reward trait, the massage, and the drink. In the entire 26 generations (11 months of real play and so many children!) I had a total of 3 sets of twins using all of the above, and no triplets.
The resulting twins were not a result of the massages or drinks, they were just random. I don't even remember if the female sim had the fertile trait because it was late in the legacy when I started seeing twins and I think I stopped wasting points on the trait because I hadn't seen any twins in 12 or so generations using the trait.
Made one guy. Made him fertile. Worked on his traits and careers.
Created 14 women, 14 houses. Made all women fertile. Worked on their traits and careers.
Started off by living with two women at a time, impregnating both (separated by two trimesters so not to fill up the house too quickly), then waited. No triplets. Moved babies and ladies out, moved two new women in. Repeat. No triplets. Repeat... repeat... repeat.
Once all households were filled with babies (7 babies and a woman), I played each of those households until the babies were taken away. Returned to main character and tried again.Then again. Fail.
Tried different strategy. Moved man into women's houses. Impregnated and waited. No triplets, then moved to the next house and tried again.
Started a new game with one couple. Played them both through careers and traits. Made them monogamous. Made them fertile. Still playing this save. Have now given away 26 babies and no triplets. Twins four times, the rest singles.
Tried making sure they overeat during pregnancy. Tried watching kids tv and putting a bed in the kids room. Tried filling a bedroom with flirty stuff and making sure both sims were "very flirty." Tried getting sims maxed on all traits before having a baby. Tried with zero traits (except charisma from chatting up). Tried potions beforehand. Tried rockets, beds, and hot tubs. Nothing works.
I have now had more than 300 babies across two saves and no twins. Probably way more. Twins about every 4th or fifth pregnancy. My wife is playing her own save. Built a massive world. Uses full cheats, so no trophies. Even she has never had triplets.
Playing on a Mac...
I've had a similar issue trying to get triplets. Both parents have fertile trait, lot has On Ley Line trait, I get a fertility massage, watch Kids TV and only get TWINS. I've done this 4 times in a row and only twins. (Household only has 5 sims so not an issue of lot being full). I've deleted a lot of toddlers now...
Has anyone figured this out yet? I have not used cheats other than building cheats and out of 25+ pregnancies after the fertile trait my sim has only had one set of twins ( I believe it was either directly after the purchase or the one right after). Tia.