Yes 3mph is the pace for slightly rough trail. On flat/smooth ground it is slightly faster. Up at altitude & scrambling the pace will drop, but the CT doesn't have much of that. The CT is high altitude but overall fairly easy grade(the CDT has much more challenging sections).
8 hrs is nothing, once you have trail legs many thru hikers will basically walk the entire day. 3*15 = 45 miles per day give or take. In CO it will be slower generally as CO is not the easiest hiking, but still 18 miles/day is very chill pace.
The CT is also only 450 miles, so when they finished, they were just starting to get in shape really. On a longer trail is it generally considered that around 500 miles is where people start to observe really obvious improvements.
"many thru hikers will basically walk the entire day. 3*15 = 45 miles per day give or take. "
Yes on the many "hike all day" thing, but regarding mileage this is starting to shade off into hyperbole. E.g., when Scott Jurek set the (then) AT speed record in 2015, he averaged 49.2 miles/day, and that was with support (van carrying food, gear, and meeting him for sleeping spot each night, people helping with massage, etc.). And Jurek is (or once was) a world-class athlete.
Heather Anderson set the "unsupported" (she had nobody helping her carry anything and no help with camping or resupplies) AT record, also in 2015, and she averaged just 42 miles/day.
The speed records on PCT are a little faster in terms of miles/day, but not much. So I would not say that "many" thru-hikers are averaging "45 miles/day give or take", although I agree that 18 miles/day is pretty average. A lot of people may hit a 45 mile day somewhere along their hike, but it's exaggerating to suggest that more than a handful average anywhere near that over the entire hike.
8 hrs is nothing, once you have trail legs many thru hikers will basically walk the entire day. 3*15 = 45 miles per day give or take. In CO it will be slower generally as CO is not the easiest hiking, but still 18 miles/day is very chill pace.
The CT is also only 450 miles, so when they finished, they were just starting to get in shape really. On a longer trail is it generally considered that around 500 miles is where people start to observe really obvious improvements.