Dear Girl,
You are 20.
Find a single guy.
Get your self respect back.
Don't be used for sex or cooking or whatever- if you think that is the deal that you are getting. Plenty of guys out there, who are not 'users'. Anyway, marriage entails both (sex/ cooking), and is perfectly respectable.
This guy is too old. Has a wife. Has a child. Is never going to leave both. - You are available anyway. If you think you are his friend- leave his family alone. If he thinks he does not want his wife- he should take that decision w/o any prompting from you. If his wife is not good to him, that is his problem. Your feeling awful is not helping your situation. Least of all none of this is helping you.
Don't ever get into a 3some. It will kill your self worth even more.
For Christ's sake get back your self esteem.
Do yorself this favour. Get out of the relationship.
You are too young to get married (20). Wait a while- there is plenty of time to get kids. Have your own- why do you want someone elses? If you end up with the wife's kid- what would she have? Minus husband, minus child?
Do you want to get to know the wife to ingratiate with the family- so that can facilitate wife's exit? Ask yourself- the motive.
No harm in friendship- be friends. Leave it at that. If you can.