These always come in packs of three. They are part of the Dust Ring. The thirst three will spawn once all Dust Devotees are destroyed.
The Triumvirate of Soil have the ability to make themselves absolutely invisible (see invis won't help) and all that remains is a little cloud of dust at the ground, showing where they are. They don't hit particularly hard, so three offtanks with a single cleric as healer should do the job.
Once the first Triumvirate is gone, the boss of the Dust Ring, A Perfected Warder of Earth, will spawn and he will bring with him another "wave" of three Triumvirates of Soil.