There are a lot of things it could be, but it would be easier if we had a photo. Many times it is enamel hypoplasia in which the outer shell of the tooth didn't form correctly and doesn't contain the mineralization the other teeth have. The odds are we would just keep it clean with fluoride toothpaste and watch it at this age. Then as the patient is older, depending on the size of the abnormality, we could line it up with the other teeth polishing it, with a filling, or a crown. The best thing to do would be to have a dentist take a look.
Those canines could be reduced slightly with a handpiece easily. You're right, they are not that bad. I would try that first, because you have nice looking teeth with no stains, crowding, or much rotation. And no, bonded teeth do not whiten like the natural tooth enamel.
Notice everyone mentions braces or orthodontics. Based only on what you've given us, if I had your teeth, I would do the same. It will be worth it in the end if you can keep the teeth intact and re-position the nice teeth that you have. If you were much older and had stained uneven teeth, cosmetic procedures would move higher up my list. But you have nice enamel, the teeth are just in the wrong places.
The old material should be totally removed because I have found things hiding underneath too many times like decay or stains. Don't worry about enamel abrasion because the new bonding or veneer you are getting will cover it and protect it as well. Your dentist will also be very careful only to remove what is necessary.
Replacing 3 teeth on 2 implants will need 2 abutments to connect to the implants, and 3 crowns connected in the bridge. @ $1000/ crown and $8-900 per abutment looks like $5600-$5800 here in Texas.
