I don't see a problem with it. Marriage is rapidly changing in all aspects, and if three or more people want a marriage, they should be allowed. It isn't harmful, and could have beneficial qualities that come with it, such as: more time with the kid, taught to accept others, more jobs for a better college etc. I think the government should stay out of marriage, they aren't needed.
No, a child should not be allowed to have more than two legal parents, because parents would find a way to abuse that system. People who do not want to be named legal parents would be named legal parents, even if they don't want to be, all in the name of tapping the other people for child support.