I'm trying to find a convention last year or early this year (as early as October 2010) that all three actors attended, where they could have signed this box set.

The premiere in NYC, maybe? They didn't do much signing at the screening (just donut deliveries! ♥), though a few lucky people at the very front of the line managed to get autographs, but there was a signing at a bookstore a couple of days earlier.