My daughter's five children have roughhoused from day one. Why? That was normal behavior in her husband's family and he has always played rough with them.
Well toddlers can be rough little kiddos when they're around that age so your 1.5 year old may just be in that stage. Because they don't ahve any concept of "hard" or "soft" "gentle" or "Rough" they don't realize they're being so hurtful to their peers.
Consistent discipline is the key to this...also, distracting your child when this behavior begins so that it will slowly phase out of his or her repertoire of behaviors. By discipline I mean you gently pull the toddler away, tell them forcefully No, and continue to pull them away if it continues to happen. after the first or second time you can say "You will go in time out for (insert length of time here) if you do not stop." because your child is 1.5, you don't want time out to be long. 30 seconds at the most, where you sit the child in a time-out spot, sit with the child, and count out loud...I would recommend 10-15 seconds, just enough to stop the behavior by interrupting it and indicate to your child that there are consequences for his/her actions but not enough to torture the tot since sitting still is awful for toddlers, lol. For my 2-year-old she gets 15-30 second time outs depending on the infraction and it works because now all we have to say is "do you want to go in time out?" and she desists whatever she was doing.
Anyway....distracting when the behavior begins will also help.
However, if you don't want to distract and/or discipline, most likely the little one will outgrow this once out of the toddler stage. My cousin's child was a rough houser from 1.5-2yrs and then by 2.5 had outgrown it and was gentle and kind again.
humm its usually the other way round , I have to ask, where did he learn it from are you saying the 3 year old doesnt do it, has never done it, how about Dad does he like a rough and tumble, with the kids I mean ...?