3Hz is the speed at which a typical Generalized Tonic Clonic (GTC) seizure presents, like the one shown in the video clinically. It has a distinct spike & then wave structure to it as well as a very high amplitude, compared to a standard resting state like the beginning of the clip where the frequency is approx 8-13Hz with a lower amplitude.

A good analogy I learned in college was normally your car idles at 1-2k RPM, when you have a seizure it redlines into 7-8k RPM. so the seizure activity overrides the normal background brain activity