Is zoom whitening good for me if i only have 3 discolored teeth? do they charge by the tooth or for the whole mouth?

Generally: Generally by the arch (in other words upper or lower) but most people have the entire mouth done at the same time. Individual teeth can often be whitened, but without examining you, i'm not sure if individual tooth whitening would be your best option. There are other methods to deal with these types of situations (such as porcelain laminates\ceramic crowns). See a local dentist for a consult.

Whole mouth: I would not Zoom in your case. Find out why the 3 teeth are discolored and address that first before trying any whitening. They may not respond depending on the cause of the discoloration. Good Luck.

See your dentist: Your dentist can determine if you are a viable candidate for partial Zoom procedure through a thorough oral and cosmetic examination. Remember, only a dentist can get your teeth this white, this fast. Ask your dentist beforehand the final cost.