Check out the rhythm breakdown below. Notice the classic "sawtooth" appearance of the rapid and regular atrial rate (P waves/ flutter (F) waves) at a rate around 300 beats per minute. We notice 3 flutter waves per QRS, giving us a 3:1 conduction. This is confirmed in the atrial to ventricular rate ratio (atrial ~ 300 BPM, ventricular ~100 BPM). Finally, note the ventricular response for atrial flutter as well as atrial fibrillation is said to be controlled if it's less than 100 BPM.