Does order matter when #defines are using other #defines?

The preprocessor does multiple runs and only finishes when no other occurrences of all defines are found. Thus, your code samples both work but the preprocessor needs one more run in case of the second one. You have to be careful with recursive defines. The ppc then would never exit.

Actually it is because of two step parser. In first step it tries to resolve all the symbol and in second step actual value is placed.

three macro need be only defined before use of nine macro. You can even change three before every use of nine:

#define nine three*3
#define three 3

int main()
{
    std::cout << nine; //9
#undef three
#define three 4
    std::cout << nine; //12
#undef three
    //no `three` macro defined here
    int three = 2;
    std::cout << nine; //three * 3 == 6
    return 0;
}

This steps would be done in the preprocessor step and all literals would be replaced with their value. This can be verified if we compile with the option:

$ g++ -save-temps basic.cpp -o out

This is the output after preprocessor step(basic.ii file):

int main()
{
    std::cout << 3*3;
    return 0;
}

for your sample program. So order should not be matter as it is kind of find and replace and it is getting done prior to actual compilation of the program.