DS3 has just started preschool and DD has just started cruising. Both relevant factors.
Low level stuff - pushing and shoving if baby climbs up whilst he is playing / doing something, lots of physical play (ie older child climbs on younger child), no outright hitting but occasional outbursts.... today DS shoved a wooden toy in the little ones face.
We have tried time outs, sticker charts, we do plenty with him alone, we praise good behaviour. He has settled in amazingly in preschool and I get glowing reports as to how well behaved he is. He is also fine out of the house and if he is with granny / aunty.
Sticker charts don't wash if you're 3. You need to use the word "no" and sound like you mean it. Remove him from what he is doing, tell him he needs to be "gentle" and "kind" repeat as necessary.
Get him to do practical things to help you, like fetch a nappy or put a bottle back on a table (if you're using bottles) etc etc. Get him involved in a way that he has responsibilities as an older brother.