founders wants me to join for just equity, how much should I ask for?
seven: I think this is very realistic.No salary is a big no go for me. This is something I could only do with close friends that I have known for years.Think about it this way: If those 3 guys really believe somehow in their project, they should be able to find some money to give you as a minimal salary.If they expect you to take a high risk (not getting any money) they should give you a high reward if everything goes well. 
To pay someone means that you take the risk that she does not produce any value, but you win the most if she does.