There is no guaranteed way to have triplets on The Sims 3.
Your best bet would be to have both parents get the "Fertility Treatment" in Lifetime Rewards. This give you more chance of having twins or triplets.
There is a way to control the gender of your baby though. Eating 3 apples gives you more chance of having a boy, while eating 3 watermelons gives you more chance of having a girl.
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on sims 3 have the fertility treatment on both the sims and then while they are showing make them listen to kids music or you will end up with twins if you do it wrong on sims 2 you will have to get a hack
What you do is when your sim is pregnant and having the baby stay at home and type Ctrl+Shift+c and type in twins or triplets if you want triplets and there you have it you have twins!!!Your Welcomelook me up on youtube as dolia3654
To get twins, you can use the cheat "forcetwins" in the cheat console after your sim is pregnant.
no, there is no murdering in the Sims
My guess is that you can't make your sim to a vampire on the sims 3.