c++ read each 3 elements in array

I am working on a project and i want to print in order each 3 elements of a string array.So if the string is "cadgfacbda" i want to be printed in the console :

**"cad gfa cbd a"**

This is the code :

        string str("cadgfacbda");
        for(int i = 0 ; i < 3 ; i++)
        {
            for(int j = i ; j <  str.size() ; j +=3 )
            {
               cout << str[j]<<" ";
            }
        cout<<endl;
    }

But what i get is :

c g c a

a f b

d a d