I like all 3 places. These are our preferred places to stay:

NYC. The Marmara Park Ave
BOS. The Lenox
DC. JW Marriott

Fun to fly into DCA but the train from Penn 34 arguably easier. I think I prefer DC to Boston, but nightlife can be quite dead there on a weekend. If you get the Amtrak down, you could also consider a night in Philly if you wanted to tick off another location. You don’t need more than a night. DC is superb for walking… 3 or 4 nights there is ideal.

It also sounds like you booked this before BA hiked the surcharges… if the OP got this itinerary reticketed would the fees jump up to circa £2k? I’m unsure of the answer.