Which of the following is correct way of concatenating two string objects in C++?

way 1:
string s1 = "hello";
string s2 = "world";
string s3 = s1 + s2;
 
way 2:
string s1 = "hello";
string s2 = "world";
string s3 = s1.append(s2);
 
way 3:
string s1 = "hello";
string s2 = "world";
string s3 = strcat(s1,s2);