You may want to review adding two polynomials first.
To add more than two polynomials, you just go one addition at a time. First add the first two, then add the result to the next one, and so on until you finish. (Actually, you can do it in any order; but if you do it in this one then you're less likely to forget one of the polynomials.)
It's the same thing if some of the operations are subtractions; just go from right to left. With subtraction, though, you have to do it in that order; otherwise you'll get the wrong answer.