Try:
string s1 = "One";
string s2 = "Two";
string s3 = "Three";
string combined = string.Format("{0} or {1} or {2}", s1, s2, s3);
Or
string s1 = "One";
string s2 = "Two";
string s3 = "Three";
string combined = s1 + " or " + s2 + " or " + s3;
But this is not good practice.