Look at this line from your code (there are 3 such lines):

s3 = s3 + s[j];

At worst, this is executed $$$n$$$ times per iteration of the outer loop. Each time, the program creates the concatenation of s3 and s[j], which could take $$$O(n)$$$ time, then assigns this concatenation to s3, which takes another $$$O(n)$$$ time to delete the old version of s3 and assign the new string. To fix this, you would want to do s3 += s[j], which doesn't re-create the string every time, or even better: s3.push_back(s[j]), which is faster performance-wise.