BWI, DCA and IAD are 'co terminals' under the city code WAS.
However, oneworld removed the ability to use 'co terminals' a few years ago so transferring with a city such as arrive DCA, depart IAD now uses up one of the sectors on the ticket.
eg, Sydney-Tokyo-Osaka is 2 sectors, but if that connection in Tokyo requires changing NRT-HND, then it uses 3 sectors on the ticket.

Flying LHR-IAD-DFW-DCA-ORD-HKG is fine and uses the amount of sectors flown.

My last DONE3 had -SEA-ORD-LAX-IAH-ORD-AMM-. I had additional off ticket flights out of ORD (ORD was just a connection point for stops in STL, DTW and DCA, but as far as the DONE3 was concerned, it was 2 stop overs in ORD). Ticketed just fine via the online tool.