All three are auras on your TITANIC and MONSTER models, buffing friendly JAKHALS or GOREMONGERS units within 6 inches, or within 9 inches if the model is TITANIC.
Idol of Infinite Rage. Models in the affected unit get +1 to the Hit roll and +1 to the Wound roll. The offensive one, and the one to save for the turn your cultists actually reach something worth killing.
Idol of Burning Wrath. +1 inch of Move, and +1 to Advance and Charge rolls. The mobility one, and almost always the right first pick, because the detachment's whole problem is getting a slow horde into contact before it is shot to pieces.
Idol of Blessed Blood. A 4+ invulnerable save. The defensive one, and the one to hold for the turn your opponent's best shooting is aimed at the cultists.
The practical order is Burning Wrath early to close the gap, Blessed Blood on the turn you expect the heaviest fire, and Infinite Rage on the turn you fight. Getting that sequence wrong is the most common way this detachment underperforms, because a once-per-battle ability spent on the wrong turn is simply gone.
Note the 9 inch radius on a TITANIC model against 6 inches on a MONSTER. That is a much larger area, and it is the strongest argument for fielding a Khorne Lord of Skulls in this detachment specifically.