A post on the Penny Arcade Website includes a low-key note announcing they are working on two free DLC packs for On The Rain-Slick Precipice of Darkness 3 and that there are also plans afoot for a fourth episode in the series (thanks nin). Here's word:

Enthusiasts of On The Rain-Slick Precipice of Darkness 3 should know that we’re getting close to finalizing the first free DLC package, “The Lair Of The Seamstress,” which we’re hoping will come out soon. We don’t know when, exactly, but apparently there’s some kind of big convention or something next month; maybe we’ll try and put it out then. Zeboyd is coming up tomorrow to work on Rain-Slick 4 and the last DLC (also free) for the current one that details the fate of the player character from the first two games.