Introducing 7month old lady to two 11months

Hello everyone!
Me and my girlfriend are going to / are currently trying to introduce a new cat to our two boys. My girlfriends friend asked us if we were interested in another cat, it's a long story why but they can't keep her and they wanted someone who loves cats and can take care of one to have her. Anyway, yesterday they brought her over, and she's currently staying in our kitchen (were living in a 2 room apartment, bedroom living room and then a big hallway but here we don't count that as a room , and the rooms are really big). The reason we chose the kitchen was that it was the room our cats visits the least.. Not sure if that was a good idea or not but I read somewhere that I should not choose a room that our cats frequent. Well our cats doesn't seem to care much that there is another cat here, the smelled the door a little at the start but that's it. No hissing or anything either so far from them. But the lady however, has hissed and growled at fur that comes from our cats. I guess that means that she is still a bit scared? However she didn't have her ears flipped back or anything, just hissing, almost looked like she was meowing before I heard the hiss. So maybe it isn't that bad? Anyway I would like some advice on how to continue! It's the second day today so still taking it slow. If you need more info about them please ask! Here's some pics of them!
This is Waffle and Marshmallow!


And this is Mali! [emoji]128513[/emoji]

Thanks in advance!

Hey! well cats are usually very insecure when it comes to a new surrounding and the new cat is likely to feel out of place for a good amount of time. i believe that every cat has its own temperament or personality,it takes a varying amount of time to adjust in a new enviroment,depends from cat to cat i suppose.i would suggest that until she doesnt come out of her shell completely and doesnt show improvement in terms of getting used to her surroundings,you should keep the others apart.since its only the second day,we wouldnt want her to be bothered about the new home as well as the cats that come with it,all at once. its a tedious process but you have to take it slow.once she shows improvement you can always introduce the others to her