Triune Cultists are a type of enemy seen in Diablo III concept artwork. They are, most likely, related to Dark Cultists, but it is not known whether they are a subtype of Dark Cultists, or if Dark Cultists are a subtype of Triune Cultists or if they are from different monster classes altogether.
The Triune Cultist looks much more tropical than their Dark counterparts who look more like medieval evangelists. They wear an attire similar to that worn by the inhabitants of Kurast.
They wear a kind of single, long cloth that covers their lower bodies. On their upper torsos, they wear some sort of Light Armor. They may wear various types of headgear that can been seen in the concept art.
The Cultists carry a whip with them with which they flagellate themselves to bring out the demonic frenzy from within them. It is not known whether the Triune Cultists are also capable of summoning demons or transforming into demons themselves like their Dark brethren. But the whips seem to assist them in channeling whatever demonic powers they may possess.
From their concept art, it seems that the Triune Cultists also chant some demonic verses and that may allow them to cast certain spells, as chanting is regarded to be related to the Occult. They are also mentioned to ring bells that may further augment their occultist powers.
It may be noted that the Triune worshipped three entities, Dialon, Bala and Mefis, the alter egos of the three Prime Evils. The concept art also shows three different upper bodies of the Cultist. Whether these are how the three orders of the Cultist look like or if they are only variants of a common Cultist type-enemy is not known. If they are indeed from different orders of the Triune, which torso depicts which order is also not discernible. But there appears to be a connection between their headgear and the insignia of the Prime Evils as seen in Diablo I manual. From the similarity, it can be said that the full-bodied Cultist worships Mefis, while out of the two torsos, the upper one appears to worship Bala while the lower one worships Dialon.
The Triune were an ancient religion dedicated to the worship of Mephis, Bala and Dialon, the alter egos of the three Prime Evils in the mortal world of Sanctuary. They recruited men and women in the service of the Burning Hells to work against the High Heavens. The existence of the Triune Cultist in Diablo III points towards the re-emergence of the Triune.
They also seem suggest that the player may get to interact with the Temple of the Triune in some way in the final game. Whether the Triune have a greater role in Diablo III's story is not known, but it seems very likely.
The Triune Cultist may be present in the final Diablo III release. But what their special abilities and skills are, are not known at the moment.
But seeing the skills of their Darker brethren, we can make an educated guess that they may be able to summon at least one type of demon, and have some kind of ranged projectile attack to complement their summons.