I'm 18 and have underbite- its only 6mm and I can align top and bottom if I wish so not that big but my ortho says 2-3 years braces- why so long?

Underbite. 1) there could be things other than the underbite they are fixing 2) underbites are very hard to correct without surgery-- be VERY happy they think they can solve for you without having you go through the pain and expense of surgery 3) if you have concerns, discuss with your orthodontist directly.
Teeth move slowly. It takes time to move teeth safely. If moved too fast you can damage the roots and/or the blood and nerve supplies to the teeth. As the teeth are moved through your jaw bone it takes time for new bone to grow. Often in order to move one tooth, they need to move several other teeth out of the way first. This all takes time. 2-3 years seems like forever now, it is small compared to rest of your life.
Underbite. A 6mm underbite is fairly significant, but it's only one of the many parameters Orthodontists link at when arriving at a diagnosis and designing a treatment plan. 1. Make sure you are seeing a qualified orthodontic SPECIALIST. Cl III malocclusions require a specialist's expertise. 2. Follow instructions to the letter. 3. Glad surgery isn't necessary. 4. 24+ months not unusual.
Takes time. Takes time to move teeth. Many variables. If concerned, get a second opinion. Make sure your orthodontist is board certified.