I am very much in favour of "You get an animal , you keep the animal" but ....
Even if you weren't thinking about taking on Cat 2 , it would appear Cat 1 would be happy with Cat 3 and your Ex
Cat 1 wins here -
More space , her own garden, a Cat Friend who she gets along with , more excercise
Your Ex is happy to take her .
Even without the better quality of life that Cat 2 will get with you , Cat 1 sounds like her QoL will improve too.
Not dissing your Cat Keeping Skills BTW
My cats really don't like me , they love DH(Female cat) , DD(Male cat) tolerate DS if he feeds them but actively sneer at me .
I am fine with it , I love the cats , they just don;t see me as being on their radar , (I have done nothing to upset these animals , except taking them to the Vet , but DD or DH comes with me , I'm the villan)
So I know what its like to do all this and they are totally un appreciative .
If there's any issues with your DCat 1 and your Ex DCat 3 can he accomodate them having their own space ? That;s the only draw back I can think of , if there's a clash then she's stuck .