Some cats simply do not like other cats. If she was the first of the 3 - she may feel threatened by the other two's presence. And perhaps the younger male more so simply because he is young and is a male. It's as if they invaded her space. She may be more tolerant of the other female because they are the same age. Some cats get over a new introduction quickly - while others never do. The best thing you can do is give her space. Provide them each with their own litter boxes, food bowls, beds and toys etc. Also, in a multi-cat home that contains this kind of stress - I do recommend a synthetic pheromone such as Feliway. It is a synthetic copy of a feline facial pheromone that can induce a state of familiarity and security within the environment, that may allow her to remain calm and comfortable around the others. It is available in store and online and comes in a variety of ways (sprays, wipes, wall plugs etc). In this situation, I recommend the Multi-Cat wall plug-in diffuser.