Should I supress the waist of a DB suit (3 x 2 button) like the way I do with SB suits?
I assume you mean, Should you ask the tailor to take in the waist to give the coat some more suppression? That's really a matter of taste. Certainly there is nothing wrong with supressing the waist of a DB coat. I think straight-sided DB coats look boxy.
Also, with 3 X 2 button DB suits, which buttons should be buttoned?
I assume you mean 6 on 2? Six total front buttons, two of which can button? On a coat like that, generally the middle button should be buttoned. If the lapel is rolled long, then button the bottom button. But if the lapel is rolled with a definite break or angle or "fold" above the middle button, then I think leaving the middle button undone looks sloppy. Very few DB coats work well buttoned either way, in my experience. I have one, but I think it was just luck.