do you think we should genuinely send him a photo of the three of us together in real life?
It would prove beyond a doubt that you're 3 different people (especially if you supply 3 personal emails to corroborate it). But it's up to you based on how private you want to be.
Another way I could think of to prove you're 3 separate people is to show that the bulk of your activity comes from different devices on different networks. You should probably talk to Fizzer about how to establish Cazza262 is not an alt.