A method of establishing the properties of a three-mirror laser, where the properties are the three mirror reflectivities and the two cavity lengths (i.e., the separations between the mirrors). Initially, a conventional two-mirror laser is designed, providing values for a laser cavity length and for power reflectivities of a high reflectance mirror and an effective mirror. Then, the effective mirror is replaced by a pair of mirrors with a separation equal to an external cavity length, and reflectivities that may be varied in tandem so that when the reflections from the pair of mirrors are in phase, the combined power reflectivity of the pair of mirrors is equal to that of the effective mirror. A quantity .GAMMA. is defined to measure the relative shift in power reflectivity between the pair of mirrors. .GAMMA. is varied over all its possible values, and various criteria are evaluated with respect to .GAMMA..