Cats are territorial and not all of them like sharing, so it can take a while for a newcomer to be accepted. Depending on the ages and personalities of the cats, it can take anything from one week to a couple of months for them to accept each other. Introductions are best done slowly and carefully.
Cats rely very heavily on their sense of smell and they need to get used to each others scents. Ideally you should confine the new cat to one room (with a litter box, food & water) and allow your cats the run of the house. This will reassure them that they are still in control of their territory. After a day or so, try allowing the new female out of her confined area for a couple of hours, whilst your two cats are placed in that area in order for them to get to know her scent before any face to face introductions. At the same time the new female should be allowed to explore the rest of your home and become familiar with the scent of your two cats. Whenever you stroke any of the cats, let the other cat(s) smell your hands before washing them as this will help familiarise them with the smell. The cats will also be able to smell each other through the bottom of the door to the confinement room. Believe me once their scents are intermingled, they will become much more tolerant of each other. When the hissing and growling has stopped, wedge the door open 2-3 inches maxium, so that they can see each other without being able to get at each other. Make sure that the door is secure and that none of the cats could force their way in or out of the room. When they can look at each other without any hostile reactions, then you can consider allowing the new female free access to the rest of your home. They should now at least be able to tolerate each other's presence, but keep your eye on them to make sure things don't get out of hand. A plug in Feliway diffuser may also help as they emit pheromones that are soothing to cats.
The web sites below offer further advice on how to introduce the cats to each other.
http://www.tulsa-animalshelter.org/tips/...
http://messybeast.com/first-impressions....
Be patient with the first two cats, they are bound to be afraid and upset by the thought that the newcomer may be trying to steal their territory. Offer them lots of reassurane and food treats when they are well behaved in the presence of the new cat and this will help them associate her with good things. Once they understand that she is an additional member of the family, and not their replacement, things will calm down.
Hope everything will work out okay. Good luck.