I am planning to have a 3 unit fixed bridge made. My question is if a bone grafting is necessary?

Unless you have an enormous defect you should not need a bone graft when a bridge is placed.
 

You do not need to do bone graft for a bridge to replace a missing tooth. Even if you have defect on your ridge on front teeth and you are not happy with it due to cosmetic, you will be able to fix it with soft tissue graft.

Almost always NO. There are different types of graft though.

But, It is not uncommon that you need a soft tissue graft. If it is a front tooth (and sometimes a back or side tooth), part of the final "look" of the new bridge has to do with how the outline of the gum (especially in the missing tooth space) lines up with the edge of the teeth. Over the years, the supporting tissues get thinner. In this area we don't need "bone" so much as just thick gum tissue.

If this is for a front tooth, make sure to visit an experienced cosmetic dentist. They can help design the best look overall, as well as they usually work with the finest ceramic labs, which determines how natural the final result looks.