getline is storing the entire line at once, which is not what you want. A simple fix is to have three variables and use cin to get them all. C++ will parse automatically at the spaces.
#include <iostream>
using namespace std;
int main() {
string a, b, c;
cin >> a >> b >> c;
//now you have your three words
return 0;
}
I don't know what particular "operation" you're talking about, so I can't help you there, but if it's changing characters, read up on string and indices. The C++ documentation is great. As for using namespace std; versus std:: and other libraries, there's already been a lot said. Try these questions on StackOverflow to start.