string[] m1 = new string[]{"a","b","c"};
string[] m2 = new string[]{"a","d","e"};
string[] m3 = new string[]{"b","d"};

string[] resultUnion = m1+m2+m3; //resultUnion == {"a","b","c","d","e"}, элементы без повторений
string[] resultSubtract = m1+m2-m3; //resultUnion == {"a","c","e"}