In the American version of Survivor, that round of voting will be voided. A "revote" will commence, with the two idol users and the player with the Immunity Necklace will be safe, leaving the remaining others in line for elimination. If the revote is tied, the rock-drawing tiebreaker is used.
In the Filipino version of Survivor, the players who used idols are only immune for that round. At the revote, they are still in line for elimination (unless they present another idol).
In other versions of Survivor, the method used is unknown.
In all cases, it is unknown what happens if only two people are eligible to receive votes.