You don't have to do a fellowship if you don't want to. People do them for various reasons. If you wanted to go out into private practice and do the bread and butter ENT cases, like T+A's, Tubes, and other uncomplicated head and neck cases, you don't need a fellowship. However, if you wanted to stay in academics and become an "expert" in a particular area of ENT you should do a fellowship. They are done after the 5 years of regular ENT residency. There are many options in ENT...
Facial Plastics
Head and Neck Surgery
Otology/Neurotology/Skull Base Surgery
Pediatrics
Rhinology/Sinsus
Laryngology
Most of these fellowships are 1 year in length, except for some Otology/Skull Base Surgery, which can be 2 years. So after medical school, you are looking at around 5 to 7 years, not including research years, of training.
Most surgical residencies are around 5 years, not including fellowship. Urology is 5 years, General surgery is 5 years, Orthopedics is 5 years, Opthomology is 4 years, Plastics is 5 or 7 years depending on whether you go to an integrated program or not, OB/GYN is 5 years. The nonsurgical residencies can range from 3 years to 4 years. Pediatrics is 3 years, Medicine is 3 years, Dermatology is 3 years, Radiology is 3 or 4 years, Psych is 3 years, Family is 3 years. All of these have various fellowships that range from 1 to 3 years...ie Gastroenterology is 3 years of a Medicine residency plus an additional 3 years of GI fellowhship. So to answer your question, you cannot be a GI doctor with only 3 years of post medical school training.
Hope this helps...