Really good fun question. Thanks Hugo.

I looked at it for a bit and concluded that if it didn't generate an error message then it should return the three groups with no 3 in them. Would it generate an error? Well, why should it? Having needs a logical condition expression, so an "exists" expression should be OK as long as it obeys the rules for all logical condition expressions in a Having clause (ie don't refer to an non-aggregated column attribute of a row contributing to the group other that one of the grouping attributes). But it took me a while to convince myself that banning "Having Exists" would be silly, in fact I wasn't 100% certain so I was relieved when the website told me I had gotten it correct.