Your daughter needs some tools. She's frustrated, but doesn't know any way to express her frustration other than smacking her baby brother.
First, she needs to know that hitting is NOT ACCEPTABLE AT ALL! That means spanking/slapping/smacking her as punishment won't work. Doing that just shows her you're frustrated, too. And when mom seems out of control, a three-year-old feels even more vulnerable and the hitting will just get worse. It's an ugly cycle.
My daughter didn't hit her baby brother at that age, but she would get angry and hit me. When she did, she was marched into her room and shown other ways to express herself. I showed her how to punch a pillow and even gave her permission to scream into her pillow if she thought it would help "get the angry out." Her favorite alternative, though, was to draw a MEAN picture. I'd hand her crayons and let her go to town. She got really good at faces of little girls crying.
She'd bring me these pictures and I'd do my best to describe what I saw. "Oh, that little girl looks so sad! It can be hard to be three and have a little brother who cries a lot." Often she'd then tell me about an event that frustrated her that morning about the baby. When she did, I'd just affirm her feelings. "Wow! That sounds like a very frustrating experience! I can understand why that would make you feel angry."
Somehow, just being acknowledged made all the difference to her. She needed to share what was going on in her mind but didn't have the verbal skills to do it. Find ways to help her express herself that don't involve hitting.
Be VERY firm about not allowing hitting anymore. If it means separating her from the baby for a time by putting her in her room or even a playpen, just do it. Let her scream her brains out. When she calms down, remind her that "We don't hit - EVER!" give her a hug, and offer an opportunity to interact with her brother in a more loving way. Can she help dress him? Play peek-a-boo? Praise her like crazy when she is playing nicely with him. A little praise goes a long way.
All the best!