What would you have left for vertical supports? If the windows are less than 2 or 3 feet apart then I fear that taking 3' out of the wall might leave you with flimsy, unreinforced concrete "posts" between your windows. IF this is the case, and you aren't widening the window openings, then you probably want to reinforce these "posts" with some steel somehow. If you have 3' or more between windows, then there is probably enough "meat" to these wall segments to hold their own.