How do you find the Lewis structure for three elements with the 7 valence electrons?

I don't know the answer to your first question because that confuses me too. But I think I can answer your second question. I think that you can't draw a Lewis structure for something like XeF2 because noble gases are inert; they do not readily react with other elements and thus can't bond. Correct me if I'm wrong but I hope I helped a little bit!