So, this might be a little crazy, and is based on a recently posted deleted scene, so take it with a grain of salt, but what about Truth as the third unknown/missing shard on Roshar? You can read the scene in full here: http://brandonsanderson.com/library/147/The-Way-of-Kings-Prime-Jeksonsonvallano
In particular, references to Truth made by Jek (aka the Shin assassin we now know as Szeth), make it seem like some god-like figure named Truth was/is a prominent figure in the Shin belief system (potentially shedding some new light on the whole notion of what it means to be Truthless). One specific quote from the deleted scene that led to this theory is "As it was, however, Truth forbade the attacking of children, women, and non-warriors". Of course, Truth and Honor are similar concepts, and it could be that Brandon has simply decided to call the Shard "Honor" in the time since this was written back in 2003.
If I've missed something somewhere and we already know what the third shard on Roshar is/was, would somebody be able to direct me to a quote? Thanks.