If my ex husband will not sign off rights for adoption, can my children use my husband of 3 years, lastname? My kids hate their dad and their lastnames. I know they can't sign legal forms with it, but can they say that their lastname is the same as mine and my husbands?

In the US a person can use any name they want as long as there is no intent to defraud. So yes, they can.

Yes they can, although it will not legally be their last name.

I chose not to do this for my child. She had MY last name, not her biological fathers. When I got married I wanted to change her last name, even with MY last name I would still have to get permission from her bio father. I thought about just using my married last name for her, but I thought down the road it would get too confusing for her... I mean using one last name, but legally having to use another. So she uses my maiden name.