As with the last two sets, Larrys and Jetpack Comics have revealed their next joint cover. It looks like we get a bit of Celestia and Nightmare Moon this time around. These were sent out in an email that you can find here.
Issue #4 doesn't ship (woops) until February, so we have a bit of time before any kind of pre-orders will pop up. We will keep you posted if anything arises!
Thansk to Evan for the heads up!