I've had that exact same thing a few times - the 3 quick beats and then back to normal. I happened to catch it when taking my pulse (didn't even feel a palp). I mentioned it to my cardiologist and she acted like it was no big deal. I think it's just another type of ectopic beat, not necessarily anything ominous (I just passed my cardiac evaluation and I'm told everything is fine). If your heart checked out okay in the usual round of tests, I don't think you have a thing to worry about.
Also, it's completely normal for your heart to speed up slightly on inhalation and slow down on exhalation. I remember when I first noticed that, it freaked me out a little bit too