R.M.
Are you serious? Your son was protecting his sister. That was FABULOUS! I think you should reward him for that, not lecture him.
He pushed the kid down and kicked his feet, that's all. He didn't pummel him into a bloody pulp and break his bones. The kid will live. And maybe the kid will learn not to push girls.
I'm not one for violence, but there used to be a day when boys got into fights and it was considered normal. Now if they even give each other a little push it's cause for extreme alarm.
If he gets into fights all the time, then you have a problem. If he protected his sister once -- GOOD FOR HIM. If he transfers that protectiveness to his future wife, she will adore him and give him lots of sex.
I think you should go and give him a big hug and strongly commend him for protecting his sister.
ETA -- Okay I didn't realize at first that the kid was three, so it's worth telling your son not to push little kids, however I still think that if this is a one-time event and it really was about him protecting the sister, it's not a big deal. Six year olds aren't that big or mature either. It's not like your son is 10.
I think the conversation should be: "Thank you for protecting your sister. But remember that Johnny is only three and you don't push little babies, because you're bigger than he is."
The kid only got pushed down.