Agree Boston to NYC is very doable on the train. 4 hours regular, 3 hours Acela.
NYC to DC is same, 3 and 4 hours. So, each hop is doable if you are visiting all 3 cities.
Another Boston-NYC option is the LimoLiner which is a "first class" bus (big leather seats, Wifi, steward/ess service, etc) also about 4 hours - most business travelers, none of the "dollar bus" crowd here
For my Boston friends, NYC connections are doable as a day trip - leave 6AM arrive 1020 have "lunch" catch the 2pm and home for dinner. Requires your provider to host, or you can get a short-term room on dayuse.com. If you are good at time management you can also squeeze in an actual lunch.....