I’ve lived in all three cities and they are all fantastic. If you want to see DC, take Amtrak down Thursday after the conference ends. It’s a short trip. Then spend Friday and Saturday visiting museums, walking around the mall, maybe catch a play one night. You can take Amtrak to NY on Sunday. If you went early Sunday morning, you could have time to choose one thing to visit in NY. Maybe a walk on the high line or visit a museum or possibly a play Sunday afternoon, depending what time your flight leaves that night. DC is very doable and I suggest going for it. If you listed your specific interests, I could make more specific recommendations.