So... Normally at least once, I'll explore every place with one character and get all places onto my map. BUT there are three more I hadn't reached yet in the game and don't know how to get to them. Would like to help here. (I have all other places, including Vault 87, on my map and am able to fast travel to any of them).

The three I am missing is the White House, and two more places on the East side of the D.C. city ruins. (They both have the same 'first name' but one is a park I think and the other is a building).

I'm guessing the subway tunnels are my passage to all three but where to start?

I guess you′re talking about Takoma Park and Takoma Industrial? I′m sometimes there without fast traveling to either of them but I can′t remember from where I head out.. Try Vernon Square East and see if that takes you to your location