3 stray cats in urgent need of care

Not so long ago a cat started coming Into my garden and me being a cat lover I started feeling it amd leaving treats outside, the cat was really shy in the beginning but it got used to me and I was able to pick it and bring inside my house, but it got desperate and tried to scape, I thought that the cat just wasn't used to be indoors.
Sadly she dissappeared for a few days and yesterday she came back with 2 kittens, all 3 of them are in a bad shape and needs to be taken to the vet, but sadly I can't afford vet and food for all three of them, I would call a animal shelter but I alredy fell in love with them and my local shelter told me they can't help me as they alredy have too many cats, if I were to call the shelter they would put them to sleep, but I'm not giving up on these three!