I need some help

I have 3 cats.

All three kittens were abandoned and my family adopted them. The first two were adopted two months apart and were female. I adopted the last one last November, so it's been over a year since the other two, and it's male.

Except for the first child, the relationship between the two younger ones is quite strained. They fight and hiss at each other, and still do to this day. The youngest one doesn't even dare to go to the bathroom and frequently defecates inside the house.

Please show me how to get them to know each other. Do I need to buy a cage?