Originally Posted by kaptan
Make sure you time it right because only one bus an hour. $.45 for the metro, $3 for bus. Make sure you have exact change for bus because they will not give you change.
Edit: The bus is also available from Rosslyn metro station but its first stop is L'Enfant Plaza so you will have a better chance of getting room for luggage and a seat.
The part I bolded is why I personally, if I were in the OP's position, would spend the extra few bucks and go with Washington Flyer. It runs every 20 minutes, I believe.
Then again, with 3 hours, the OP could conceivably make their connection anyway...but I wouldn't chance it, personally.