new cats aggression

Hi please could someone help me.
I have a 9 year old bengal neutered male cat called tom, 4 months ago i took in a one year old spayed female cat called mimi from an animal shelter, they got on great and bonded so beautifully, always playing together and their were never any problems. One month ago i took in another 4 year old female spayed cat from the same animal shelter called tasha, and i am having many problems. I kept tasha in a room seperated from tom and mimi for a week, then started to let tasha out to wonder around the house when tom and mimi were out to spread her scent. Then i introduced them all slowly a little each day, tom went into major spraying mode all over the house, and mimi kept hissing at tasha, tasha seemed quiet and as though she wanted to make friends with both of them, curious, and interested, we were told that tasha had lived with two other male cats previously. on first introductions tom went weird and started to transfer his aggression and frustrations about the new cat, out on mimi, attacking her nastily, both mimi and tom both seeming very unhappy that another cat was in their territory, so we had a few weeks of having to seperate them and watch them, things finally settled down with tom and mimi, so that they could be trusted together again, however, now that tasha has started to settle and find her feet ie wandering around the house, and starting to go outside, she has started to stalk tom, really charging at him aggressively, and attacking him hissing and clawking at him initiating a fight, tom just runs, and is now very nervous and edgy, but tasha is relentless and she charges after him, i tell her she is a naughty girl in a firm voice, and put her back in her room for the day, then try again the next day to reintroduce them, but things have just gotten worse, now every time she sees tom she is charging at him in full attack mode she appears to hate him, its as though she dosent want him in the house or garden, she dosent do it at mimi, although mimi is a happy little soul who just stays out of her way. after the attacks tom takes it out on mimi, and its just becoming one major nightmare. Tasha is very strong willed and hates being picked up, and i myself have received a few painful claw swipes and hisses trying to put her away in the room after she has attacked tom. I am at a loss at what to do, as i love all the cats very dearly, tasha is so loving to me when its just me and her, and cuddles on my knee. Will i ever be able to get them to be friends? and if so how? as its kind of been a nightmare this last week, today alone she attacked tom twice, and he was so upset he was literally quaking.
someone please help, i dont want to part with tasha, but i dont want my lovely tom who ive had for 9 years living a miserble existence either, and poor mimi getting the brunt of it all.

hi
i also wanted to add that i have had feliway diffusers plugged in, in every room of the house for the last few weeks too, but they dont seem to have helped.

Hi
thankyou so much for your response. I have a total of 5 feliway diffusers plugged in around the house, but they dont seem to be helping thus far,also financially i just cannot afford anymore at the moment as they are £32 each from our vets. I have toyed with the idea of giving her back to the animal rescue centre as its in their contract that you have too if you dont keep the pet, they are a great place,i have also adopted several rabbits from them, and they love their animals they never put a healthy animal to sleep, and go all out to rehome the animals, also the unrehomeable ones get lifetime homes with them. However i love Tasha to bits, and feel so devastated and heartbroken at the prospect that it makes me feel physically sick, i really dont want to do it. If you try to rub a towel on Tasha she goes mad and spits at you and claws you, which is painful and upsetting, as i hate having to tell her she is a naughty girl in a firm voice, it upsets me more than it does her! since she settled she has on occasions attacked me if the mood comes over her, which has made me feel weary at times to approach her, im trying not to let my feelings and emotional attatchment rule my decisions, but its so difficult not too. Am i selfish to want to keep her? i dont want to ruin tom and mimi's happiness in the process. I have put her back in the lounge for now. kind regards jo

hi
also its not possible to scent transfer on the cat beds, as Tasha has never gone in her bed and prefers to sleep on the settee, tom very rarely goes in his bed as he prefers to sleep on my bed, and mimi sleeps on my mothers bed, but i will try again with transfering scent through brushing.

Thankyou so much for your response, and your kind words. This morning i spoke on the phone to the rescue home tasha came from to seek help in settling her, and unfortunately they insisted that i return her so they can re-home her as an only cat, i truly did not want to, but it is in the contract with this particular rescue home that they have the right to remove the cat after adoption if the home turns out not to be suitable, so the lady who owns it had the last say in the end. She was very kind and understanding, but said that she felt this could not be fixed and things may escalate to the fact that tom would eventually leave home, and it just was not fair on tom and mimi to continue on the way we have been. So sobbing my eyes out i took her back this afternoon. I am devastated, absolutely heartbroken, and im missing her so much already, i wish she could have gotten along with the other cats, she would have had a lovely home here, i hope with all my heart she gets a lovely home, as with all her quirks she was still a lovely cat, who il always miss terribly.