Also the same number as "Hey, Siri". And Samsung's "Hey, Bixby". I'm assuming it's not a coincidence.

I think it's likely that three syllables just happens to be a nice cut-off point that minimizes the number of spurious activations, and that explains why everybody doing voice control uses that particular threshold to activate their digital assistant.

Heck, even Star Trek got it right way back in 1966 with its "Computer" wake word.

About the only company I can find that seems to be ignoring the three-syllable rule is Microsoft, who use "Hey, Cortana", which is four syllables. But Microsoft's always been a fan of using really long phrases to identify their products, so maybe that shouldn't be a surprise.