I'm really sorry to read this, I can't imagine how heartbreaking it is,

We were in a very similar situation, 2 small slightly older girls (5) and 1 rambunctious little boy (who quickly became a very big boy), we probably messed up thinking we would need to protect the kitten from the girls, ended up exactly the opposite. (Our final solution, and reason for writing this anecdote) We bought squirt guns and guarded the favorite victim like she was the president, it's taken months, but he's finally starting to get it through his skull that we don't need to play 24 hours a day. My wife has the patience of a saint, and fortunately has gotten to work from home a lot.

I hope you find peace, we can't rescue all of them.

Sometimes we are not the ones to help the ones we love the most.

I know no one forced you into this third adoption, but you got delt a shit hand. This is one of those hard-hitting stories where there isn't even a proper lesson to take away from it. You did something good for yourself and good for another creature, but what should have been a happy occasion soured and endangered your pre-existing commitments. You seem like the kind of person to throw out a lifeline to people in need, but when you needed one, none were to be found. What should you have done? Let this cat's behavior drag all of you down into misery and fear?

This experience and memory is going to haunt you for a long time and you should be grateful when it does—it means you still have the ability to empathize. I hope your neighbor finds a good friend in the cat that could’ve been, and unfortunately, he's still technically your responsibility.

If the third cat does end up back in the picture, look specifically for cat rescues in your area and let them know that your situation is dire; they'll usually do everything in their power to re-home your cat if they know that the current situation is endangering other cats.

This is also one of those horrible experiences where someone will one day tell you about some simple solution that they can't believe you hadn't heard of, and it's gonna rip the wound open again. You'll just have to keep reminding yourself "I love all three cats, but one cannot ruin the lives of the other two. I did my best in that situation, and this renewed pain means that I still have empathy."

I'm so sorry that you're in this situation and I sincerely hope that a solution presents itself that gives you immediate catharsis. I wish that angry boi the best and I hope he finds another loving home that has all the right conditions to satisfy a furry ball of aggression.

Don't beat yourself up for trying something

Some are just far too gone and need to be street. You can put some food outside for it, but that's how things are sometimes. You're still a good person for giving it a try