This is how to get triplets ( one girl and two boys).
1. Get the girl sim Pregnant
2. Get her the family-oriented trait, and the fertility treatment
3. Wait until she knows that she is pregnant
4. Ask a sim above level 5 in the medical career to determine gender of baby
5. If it is a girl, have your sim eat a lot of apples, or if it is a boy, eat watermelon.
6. Have the pregnant sim watch Kids TV and Listen to Kids music
I hope this helps!