I can't understand that why is the output of this code 15, 4, 6

Question

I have written this piece of code, but the output of this code is not what i expected. Can anyone explain me how come 15, 4, 6 is the output.

#include <stdio.h>

int main ()
{
    int a = 1, b = 1, d = 1;
    printf("%d, %d, %d", ++a + ++a + a++, a++ + ++b, ++d + d++ + a++);
}