A back tooth can have 3 tooth roots. so, the dental implant itself is one "root" for that tooth replacement?

Exactly: For that reason, if the implants being placed are short it is often suggested that TWO implants be placed to replace that missing molar. It all depends on the bite, opposing dentition, occlusal forces, etc. Yes, it adds to the total cost of your care, but you're also getting a stronger and physiologically equivalent result. Consider it. Wider implants can be used if there is bone volume.

Molar Dental Implant: A molar dental implant is an artificial single tooth root that is surgically anchored into your jaw to hold a replacement tooth.

Implant: Ideally we would put one implant per root,. but besides the cost involved,factors such as the size of the implant, as well as the surgical space and restorative space, distance between teeth , dictate and necessitate the placement of one implant. this is not always the case but the majority.

Dental Implants: Can't compare apples and oranges. An implant is a large titanium "root" replacement that integrates to the bone without any movement in bone. Roots are smaller and attached to the bone with a ligament that allows for movement of the tooth. An implant is therefore stronger in many ways and more than adequate to support a single tooth restoration.