I have 3 cats and one is very aggressive

I have 3 cats. two of them are siblings that I got as kittens. They're btoh 2 yrs old now.
In August, we found an injured and starving stray. We took him in, got him fixed up and neutered and decided to keep him. His name is Purgatory.
He's generally a nice cat. However, he has this nasty habit of picking on the other two, especially BeBe. Purg goes out of his way to find BeBe just to beat on him. BeBe doesn't fight, ever. Even when Purg has him cornered, he just lays down until it's over.
I was hoping once things got calm (we kept getting flooded out so we moved the cats around a lot) Purg would calm down. But he hasn't. If anything, he's gotten worse.
The vet can't find anything wrong with him, so I'm convienced it's all behavioral.

Over the weekend, we found a kitten, so we took it in. We put the baby in a large dog crate in a room that we could shut the door so the other 3 couldn't pester him. This REALLY upset Purgatory. The first day he ran around the house, yelling, hissing/spitting, growling...He hit me a couple times even. Sometimes I'm scared of him. I forgot to change my clothes after handling the kitten, and Purg came over to cuddle, well he started to sniff me and got ALL upset. When I tried petting him he hissed and swatted at my hand. So I said "Purg, its okay its just me..." and tried again, but he screamed and ran away.

I don't want to rehome him. we decided to keep him and I want to make this work. Friday I'm going to buy some Feliway and spray it around the house to see if that helps any...

But if you have any suggestions, I'd so appreciate them!

Definitely get the Feliway. I agree that it's behavioral and probably the flooding situation made it worse. You know how moving around makes them but obviously that was the only choice! Adding the new kitty will be a challenge. I went through the same thing with Kit when I found Seti. Fortunately, Hemmy is the boss so he helps mediate some. We've found the Feliway to be a tremendous help and keeping everyone separate for a while and doing slow intros helped too. They still aren't the best of friends but things have calmed considerably. There are some really good articles around this place about how to do intros. It's my opinion that often nervous owners totally overplay them so staying relaxed and trying to be the "calming force" is my best suggestion for them!

Well we found the owner of the kitten. I called her about the lost ad she had and she didn't think he would have traveled that far but sure enough, soon as she seen him she was all "OMG that's Tigger-Tom!" and took him.

I think I might seperate the 3 cats again and slowly re-introduce. Maybe bathe them all as well.

He's getting worse though. He attacked me once, my BF twice, and a guest that came over. The guest picked him up because Purg was trying to run out the door.