I carry two iPads, an Air for MF 3.X and a 2 for AirPlay. I used to use both iPads for MF (docked and remote) and an iPod Touch for AirPlay. And that’s more than enough extra gear for me to schlepp around.
I think, and I’m not too sure on this, but if you are lugging around tons of gear for big venues, 3 iPads might be a little much sometimes. I do small gigs, so this is not an issue. But as I get older, even lugging around the gear I use gets tedious sometimes. Playing the gig is the best part for me, that’s why I keep doing it.