5 USC 3317 - Competitive service; certification from registers

(a) The Office of Personnel Management shall certify enough names from the top of the appropriate register to permit a nominating or appointing authority who has requested a certificate of eligibles to consider at least three names for appointment to each vacancy in the competitive service.
(b) When an appointing authority, for reasons considered sufficient by the Office, has three times considered and passed over a preference eligible who was certified from a register, certification of the preference eligible for appointment may be discontinued. However, the preference eligible is entitled to advance notice of discontinuance of certification.