//functions.h

float x1;
float y1;
float z1;

float x2;
float y2;
float z2;

float ThreeDDistance(...);
void Calculate();
void GetInput();




//functions.cpp

float ThreeDDistance(...)
{
    ...
}

void Calculate()
{
    do{
        GetInput();
    }while(...);
}

void GetInput()
{
    cout << "enter blah blah";
    cin >> x1 >> y1 >> z1;
    cout << "enter blah blah";
    cin >> x2 >> y2 >> z2;

}