I haven't looked, but I doubt if there are many flights running from IAD or BWI to DCA. I live in the area and they all serve roughly the same area. DCA is my closest airport and if I flew in or out of BWI or IAD, I wouldn't think of trying to catch another flight to DCA. Once at any of those airports, you look to ground transportation. If you are commuting to DCA, plan on trying to fly into DCA. You could find ground transportation from the other two, but it would be painful.