I'd probably pay the difference to change to fly into IAD. You can try to talk them into the change, waiving the fee, but I doubt it will work. Still, doesn't hurt to ask. But I also think 3 hours to get from DCA to IAD should be manageable, though a bit stressful. I'd walk out and take the taxi. Main variables are (1) any delay with checked luggage at DCA, (2) traffic, though if rush hour, 66 will be HOV only, and once you get to the dedicated Dulles lanes, you will sail through, and (3) check in lines and TSA lines at Dulles. As to the latter, having priority will get you to the quick UA priority check behind the main ones. You might want to buy that. Precheck will make a difference, too, though those lanes can fill up depending on the time of day.