I am not sure I understand your question. An example would help a lot.
My best guess is that you are asking if you can first add (or subtract) just some, not all, of the fractions by finding their common denominator, or if you have to find the common denominator of all fractions at once. Well, the least common denominator is the least common multiple (LCM) of the denominators, and LCM can be found in any order, just like addition or multiplication. For example, LCM(x,y,z) = LCM(x,LCM(y,z)) = LCM(LCM(x,y),z) (this property is called associativity). Therefore, if you need to add three fractions x' / x + y' / y + z' / z, you can add the first two fractions by finding the LCM of x and y and then add the third one by finding the LCM of LCM(x,y) and z, or you can add the last two fractions first, or you can add all three by finding LCM of x, y and z at the same time.