so i just completed my first year at mac....but im short 3units because i dropped a course from second semester...i am going to be taking summer school next year (summer 2010) for a 6 unit course....can this 6 unit course make up for my 3unit dropped course?

Yup. Why not? You would have an extra 3 units as well.

Unless you dropped a mandatory course. In that case, you will eventually have to make it up.