The 2-3 age group is such a difficult one! I hope that you found some right middle ground for your little guy.
They are right into pushing boundaries as they are trying to figure out where they fit in with the world

it sounds like he wants her attention. or yours.

I see this is an old question and hopefully not an issue anymore.
Reading your question I think a 3 yr old can’t reason yet, let alone recognise his feelings. Sounds like he could be jealous, in which case I wouldn’t discipline him (of course you do tell him this is not what we do and make play rules), but make sure you coach him in right behaviour and spend enough one on one time with him so he knows he’s loved and special.

Very good! It was so interesting reading the comments on this!

Little boys go through all sorts of stages, he may just need a little more firm social modeling, me may just be confused about how to act appropriately around some people

Tell him she doesn’t want to see him because he keeps hurting her. He may then realise that he does want her around & to have her there he might stop

Have you asked him why he behaves like that with her. Hope you managed to sort it all out.

It is just a stage he is going through keep them apart for awhile until his behavior improves

i think he could be jealous .

It might be jealousy. My now three year old gets nasty sometimes as well and I make him stop what he is doing, look at me eye to eye, tell him he is being mean or his upsetting someone and make him apologise. If he continues I ask him to sit on the step (usually we are outside at the time) and he normally comes good. It took a few times before he was ready to listen but I stayed consistent and am now seeing improvements.

Is it only your niece he does this to? Talk to him about it and see if there is a reason he targets her. Let him clearly know this is not OK and it hurts people!

i hope you’ve had success

Hope you managed to find the answer you were looking for.

Talk with him and say that you wouldn’t want the same thing your doing to be done to you

How is your son now?

Some good advice below – has any helped?

He needs to learn to share better. I think you should consider taking his favourite toys from him each time he displays this behavior.

Without fail a time out every time he does this.Also tell him a boy does not hit a girl and start taking items from him if he continues.No TV etc

Talk to him and tell him he hurts people when doing that and they are sad. Also role play and show him its not nice

I hope his behaviour has improved since you asked