There is an easier way to triplets you have to have your sims try for a baby multiple times even after you hear chimes. Then through the pregnancy have sims Woohoo at least twice a day. Certain Twins or Triplets. NOTE this only works providing there's Six or Less sims in the household as you cant exceed 8 sims in a household.
I just tested this. I placed a male and female sim in a household. Neither of them had the Fertility Treatment, and the mother-to-be never watched any kind of television. I used Try For Baby three times, and each time I heard the chimes.
I was kind of skeptical about the "woohoo twice a day" part because I didn't see how that fit into the equation, but the sims did so anyway. At least it gave a nice mood boost.
When the female went into labor, she gave birth to a single baby boy. I think that it can be concluded from this that it doesn't work, but maybe I'll test it out again a few more times just to make sure.
Also, please forgive any mistakes in spelling and grammar; I'm using an iPad.