Atlanta is a hub so all three should have a lot of flights. BWI is a pain to get to, transferring between multiple service providers (Metro to MARC or Amtrak), so I'd eliminate it just due to being farthest away and having the worst public transit options, leaving IAD or DCA,.

A car to IAD should take about 30 min BUT could be longer due to traffic, especially on a friday at 5pm. If you use Google Maps you can enter a departure time and they mine their data to give typical transit times. Google maps says its 30 minutes to 1 hour. I don't know about you, but I'd prefer knowing I have an approx 1 hour commute but it's consistent over an anxiety producing car ride. (There is a small but nonzero chance major congestion could make this longer)

Going to DCA from Rockville on the Metro takes about an hour. (About the same amount of time as a cab) and costs significantly less. Maybe you can get brownie points for being frugal?

Just be sure to transferr in Farragut Square. M-F You can walk straight out of Farragut North, across the street, and down into Farragut West - just be sure to obey the crosswalk since they have been stationing cops to hand out jaywalking tickets

There is often a large number of people who get off at Farragut, so in addition to saving a few minutes not doubling back from Metro center, you often can snag a seat for the second leg.

Since you have precheck, leaving at 5 and arriving at 6 for a 730 flight should be sufficient.