When I go for twins or triplets and I want to lock them in, I will at the twirl eat the 10 or more of whatever fruit is needed.
Generally, I get same gender that I plan. For twins or triplets, unless I need a certain gender, I just let it go and see what happens, it is probably a fluke, but I have had my sim eat seven apples, then eat six watermelons, then five more apples, then six more watermelons and had mixed gender twins, but like I said, it just might be luck of the draw, instead of my locking anything in.
I had read that the eating of the fruit had to be after the twirl, before and it won't work. I have a couple of times had my sim eat the fruit before the twirl and not got what I wanted, the male or female baby.
As for triplets, that is pretty much a guarantee if you watch tv or listen to music, especially if you do both. As for procreation elixir or the fertility treatment, I have never had any luck with the elixir.