How do you house train a 3yr old male small dog?

I was given a small dog that the people were going to have put down if I didn't take. The ole guilt trip. Well I took him. He's not totally housebroke and it's driving me nuts. I crate him at night, he goes outside with the other dogs several times a day but I'm still finding pee here and there. How do I get him to let me know when he has to go out help help help