What REG says that it is a REQUIREMENT that a Soldier stays on an installation for 3 yrs?
Basically that the Secretaries are responsible to keep PCS' to a minimum and they should keep a record of PCS'. Read 4.3 below.
It is DoD policy that:
4.1. Assignments shall be made for all Service members without regard to their color, race, religious preference (except chaplains), ethnic background, national origin, age, marital status(except for military couples), or gender (except where prohibited by statute and limitation of facilities) consistent with requirements for physical capabilities.
4.2. Reassigning a Service member, involving a PCS move, shall be authorized only for national security or for ensuring equitable treatment of Service members.
4.3. The primary considerations in reassigning a Service member shall be the Service member’s current qualifications and ability to fill a valid requirement. Other factors such as availability, volunteer status, TOS, and other criteria shall be secondary. When Service members with the required qualifications are identified, then the other factors and criteria shall be considered. Military Services should try to minimize personnel turbulence.
4.4. Specifics concerning DoD policy, responsibilities, and approval authorities for selecting, training, and assigning personnel to Security Assistance Organizations are in DoD Directive 2055.3 (reference (d)).