Both Jesse and Joey were in and out of work. Jesse was an exterminator, then a musician, a commercial jingle writer, a club owner, even that hit he had in Japan he was probably getting jerked for his publishing. Joey was a weak comedian who parlayed that into a show on public access, they weren't making money on a consistent basis. They helped Danny when he was at his lowest, the least he could do was give theme a roof over their heads...
Or the three of them were running a child porn ring and using the girls as material
I rather go with the first option, my childhood can't handle that second one