What is the exact buff given by the trio of Demon pets?

Having one out gives you 3 attack/magic attack, having two different ones out gives you 3 + 5 attack/magic attack, and having all three different ones out gives you 3 + 5 + 7 attack/magic attack.

This bonus is also given by the Shark pet and Pink Bean pet.

Lovely, an extra 15 attack. Thanks.

Are you sure about the Shark pet? I have one and I don't recall it buffing me. Although, I can't log into the game right now to check.