Next year -- since they updated the Mini, and the Pro was announced (even though it won't ship until November), that tells me no Air 3 this year. If there was one, wouldn't they have announced it with the other two? If there was an October event, they could have did iPad Pro there, since it's not up for pre-order, and doesn't ship until November anyway.
My bet is WWDC 2016 for an Air 3. Macs will probably be updated early next year, so maybe they could shoehorn it into that announcement, but I'd not be surprised to see it go longer.
They did call the iPad Pro their new flagship iPad, so perhaps they want to give it some time with higher end specs before they move that hardware lower down the model line.