I think I kind of get the idea, although the explanation still confuses me. But if I read this right, there are four criteria: Number, colour, shape and fill. 3somes have to have only one of those in common, the other three criteria must be different. So three square cards must have different colour, number and fill. Similarly three cards with two items on them must have different colour, fill and shape. Those are 3somes.
I don't know if I've encountered any where ALL criteria are different or the same, which is what the directions had me looking for. i.e., all four criteria match, or do not match.