That's a rule of three problem. Nothing to do with molecular biology. It's easy.
3mM NAD / 1000ml
(that's your final concentration, 3mM per one litre)
Now you want 2 ml of that, so you do a rule of three
( 3mM NAD / 1000ml ) = ( x / 2ml )
(you multiply your 3 by 2 and divide that by 1000)...