OP, right now the cardinals are out singing a lot -- they don't have the range of a mockingbird (which can mock anything), but since you specifically mentioned three tones, I thought that could be it.
Cardinals have a few distinctive calls and are common in the DC region. They sometimes start with a call that's three tones (almost sounds like "burrito! burrito!" to me!)

Then, they do "cheer cheer cheer cheer" (descending notes). And a "whoit whoit whoit whoit" call that's very cool and almost outer-spacey sounding! They mix these up and really put on a nice show in the spring, and I think, are one of the most beautiful singers in the bird world.
http://www.youtube.com/watch?v=C9LNexIoCW0&feature=related
The video has "cheer cheer cheer" at about 2:00 and not super-clear recording of a female singing "burrito burrito burrito" at 1:45.
There's also:
http://www.birdjam.com/birdsong.php?id=3 Which has a great clear "cheer cheer" part, and the cardinal's "alarm/alert" call (a little metallic "chip! chip!").