3 year old son sleeping in same bed as mother. He has his own bed to sleep in in her room but she makes him sleep with her. So now when he comes to his dads house its so hard for us to get him to sleep in his own bed in his own room. I live in Wisconsin. Is there any laws I can lay down on her to make this stop?

No there are no laws against co-sleeping. Many parents do it.

So there's nothing we can do?

Just parent him at your home. Make him sleep in his bed at your home. But no, there is nothing you can do to his mother because she has chosen to co-sleep with her son. It is not illegal.

We do our best but it gets more difficult every time

We do our best but it gets more difficult everytime
What kind of bed does he have at your house? A boring twin bed? Maybe make it a special experience -- put a mattress on the floor and hang a sheet over it like a tent and let him go camping with a flashlight and some books to read. Whip up a story about his camping adventure, read with him for a while, and then leave him to enjoy his camping experience alone.

There are other things you can do if that one isn't your cup of tea. You want him to WANT to sleep in his own bed.