My first triplets were the blue-haired girls Moonlyght, which you congratulated me on earlier Thanks by the way!

Some couples are more likely to produce children that look exactly like them or other children that all look alike depending on where they are located on the head chart.

I also had rainbow-haired twins which are seen in the same screenie above.

I don't think a certain head will more likely produce a twin or triplet, it depends on how frequently you choose those peeps which is your preference.