My family and I have had 2 cats since kittenhood for 5 years, Ernie and Dolly, male and female (brother and sister) and have just a few weeks ago taken in a lost, probably abandoned female cat whom we named Lucy. We are inexperienced with taking in an adult cat into a home with other adult cats. So far, they are appearing to not get along too well, but on the other hand I’m not sure what to expect to begin with. For the first few days, we had Lucy separated into a room where she had her own litterbox, food, and sleeping places. After awhile, we started leaving her door open to let her roam the rest of the house. At first my resident cats hid under the bed, as they would either go under there themselves upon seeing her, or Lucy herself would chase them under there. They’d stay under there until we put Lucy back into her original room. This is something that my mom has gotten into a routine of doing. She’ll let Lucy roam the house for a long while, but will then put her back in her original room, because she feels sorry for Ernie and Dolly because they won’t freely roam the house till they know Lucy’s in her room with the door shut. They either go under the bed during Lucy's out time, or they sleep or hang out on high elevated pieces of furniture. I had an idea to at times, put Ernie and Dolly and Lucy into Lucy’s original room together (with me supervising) so they could get more exposure to eachother. I first tried it with Ernie and Lucy. In the room, after some growling and yowling mostly from Ernie, all seemed ok since they were then on 2 different pieces of furniture and were quiet…but then all of a sudden a fight broke out somehow (which I broke up) during one of the times I was looking away. They then stayed looking at eachother for hours under the bed and desk with Ernie growling intermittently at Lucy. My main question is, is what my mom and/or I is doing a good idea? If we’re patient, will they eventually at least tolerate eachother (all 3 of them) at worst, if not become friends, at best? What should we keep doing and what should we stop? What are some good signs to look for amongst the 3’s behavior towards eachother? Advice will be soooooo appreciated! This is getting depressing for my mom and I to see cuz we just want all 3 to be happy together. lol.

Thanks in advance and sorry this is so long.!

Don't break up fights. They're trying to sort out their hierarchy.

Breaking up a cat fight is dangerous for you. It's normal for cats to fight when they meet a stranger and since they're living in the same house they need to establish hierarchy. If the fights are overly aggressive (blood, screaming, etc.) you can separate Lucy into her own room again and start from scratch.

Generally, the best way to introduce cats is with patience. Leave the new cat in the room until the other cats stop growling/hissing so much (and leave the door alone). Letting the new cat roam the house and the old cats into the new cats room is a way to see what the old cats think and get them used to the scent. Getting three baby gates makes the next step easier as you can place all three in the doorway and reduce the possibility of fighting. Leave the door open when you're home (baby gates in place) and let them sniff and inspect each other.

Once the cats stop watching and hissing/growling at the new cat through the baby gates you can try letting the new cat out into the house supervised. Lots of high places, several litterboxes in good, open places, multiple food areas, etc. help the process along. Getting Feliway or Rescue remedy (I think that's what it's called) is great for relaxing tense kitties.

Aaaand... I think that's it...