founders wants me to join for just equity, how much should I ask for?
yogibear: Everyone -- thanks a lot for all the answers and insights.
That helped. 
To give you more details, since some are curious, they are just beginning, about a month or two. There is some code but its not more then 5-10% of what is needed for public beta.One other advice I got is to strive to stay with ~3-4% after all dilutions, which should be estimated  as being diluted 1/3 for three times. so to stay with 3% i should start with 10.5% (->6.9% => 4.6% => 3%).One other detail I didn't mention is that they want me to keep my day job (me salary paying job i currently have), until the first round (seed money). 
Currently the three of them also keep their day job, so we are all kind of equal on that aspect.Regarding the founders abilities, they have design/biz dev/ceo experience.