Since you don't know what cards are also out, you can only discount the cards you have seen. The deck is shuffled and dealt randomly so the chances of getting the card you need are assumed to be the same.


If all of your needed 3's were dealt out to other players, there's a zero percent chance to complete your draw of course but that doesnt matter. Over the long run, the chances of hitting your draw stays the same. We can only use what information is available so that is the best way to ensure long run success.


( I hope someone more gifted with words can reword this for me, because I suck at explaining things.)