Well thats how I feel. He can play really well with other children, and then suddenly from nowhere he hits. Actually it isn't always completely from nowhere - it could be because of a tussle over a toy, but sometimes I feel it is just to get a reaction from the other child. He hits his little sister too (although he can be lovely with her). The little sister hitting is very often related to him trying to get my attention e.g. during a nappy change, or it can be because they are both tired and she is crying.
Sometimes I feel that every interaction with another child is so fraught because I am so worried that he will hit or push them. He can spend a whole day with other 3 year olds without incident, but I just hate it so, so much when he hits another child. If this happens in a public place e.g. soft play I will usually take him home immediately. Maybe there are other 3 year olds who do this, but I can't say I've seen them and its quite lonely.