This is SOOOOOO normal and nothing to worry about. THere's no big secret that having a new sibling takes away from the first/only child and that, eventually, the "only" WILL act out his feelings about being displaced on his younger sibling. At age 3, he'll be "sneaky" about it and wait for an opportunity when no one is looking as he is invested in you thinking he is the ALL LOVING BROTHER. He's only human and he's a little human being at that, with little or no impulse control when it comes to pain as deep as being displaced can cause, so the ONLY thing you can do is be aware that, no matter how loving he is most of the time, he CANNOT be trusted alone with the baby, period. It's not fair to him to trust him with it as he will, at some point, like all other siblings, feel the need to act out on his hurt over being displaced by that little baby and he won't be able to stop himself so you have to prevent the sitation by not allowing him the opportunity. I know that's SO much easier said than done but it's no different than when you have to juggle things to make sure baby is in a safe position when not in your arms....you just have to make it as forbidden to leave the older one alone with the younger as you would make leaving either of them alone in the house.....just not acceptable....
In the meantime, DON'T panic or over-react to what your son has done....it's a very very normal and common thing and a good opportunity for you to talk to him about using his words to express his feelings about his baby brother being mean. Threes need lots of words and practice using those words when it comes to emotions etc. as they are so full of new ones....especially if a new sibling has entered the picture. You don't want him to be overly alarmed by his behavior toward his brother or he'll think he's bad and the baby's good (three's tend to think in black and white)and his self-esteem will be negatively impacted.....but just a clear and factual and properly emphasised, "OH no, we don't hurt people when we're angry with them, we use words to tell them and if we have to hit something, we hit pillows...." something along those lines. Your little guy LOVES his brother AND he resents him a bit, with good reason so he's going to act like a kid and act out BOTH his love and his resentment and you have to be on the alert to it and prevent an environment conducive to him having opportunity to be alone with his brother.
I've been a professional nanny, teacher and early education specialist for the last 20+ years and I've seen the most loving 3 year olds in the world who ADORE their siblings lay in waiting til mom turned around and hit the baby hard and fast. They ALL do it.