Poor little stray kitty

Outside my house is a stray gray and white kitty who desperately needs a loving home, I believe he/she (not sure of the sex of the cat) was a house cat and someone abandoned it :( , I would let it in my house except for the fact that it hisses at my two cats I already have. So, I have decided to construct a sturdy house for it outside thats insulated because I can’t bare to see it struggling as it gets colder and colder everyday. I will NOT take it to the spca because I do know the sad chance that the cat may be put down if no one wants it and I will not allow that. But seriously it needs a warm home, if anyone could take it in that would be great!