Sort multimap elements of the same key based on a class member?

I have a class S which is stored in a std::multimap<int, S>:

class S{
    int _secondKey{0};
    int _thirdKey{0};
};

I would like to store elements of the same key, sorted based on the _secondKey class member and then _thirdKey member.

Is this possible to do in C++? I am using GCC 5.3