<human>: Can you succinctly summarize the text in 1-3 sentences?

We have three cats, two of them are about 4 or 5 years old, and the third is about 2 years old. We got the first two (male [referenced henceforth as Mcat],  and a female [Fcat]) around the same time and they got along fantastically, sleeping together and licking eachother etc... A few years later we decided to rescue another cat from the pound, and brought her home with us [Fcat2]. Pretty much the same week, Fcat and Mcat no longer got along. They don't fight, but if Fcat ever leaves her house in our cat tree Mcat will chase her back there. Fcat is also scared of playing with Fcat2 for the same reason, even though she doesn't hiss at Fcat2 or anything. All three are spayed/neutered. Fcat pretty much only leaves her cat tree if Mcat and Fcat2 are definitely in my room, and this is when she eats and such as well. She's still a very lovable cat, she will stay on our laps for affection if the other cats aren't around. We'd just like to get Mcat to stop chasing her so she can feel free to wander the house again. Any tips or advice? If we can't help her in the next few months we're thinking it might be best to give her to someone else if we can find a suitable home (we wouldn't send her to the pound)
<bot>: Two cats no longer get along after introducing a third younger cat, need a way to get them re-adjusted to each other.