  3:2 and/or 3:1 twin high frequency quasi-periodic oscillations (HFQPOs) have
been measured for the three microquasars GRO J1655-40, XTE J1550-564 and GRS
1915+105. For a test particle orbiting a rotating black on a stable circular
orbit there exist two different orbits at which the vertical and radial
epicyclic oscillations are in either a 3:1 or 3:2 parametric resonance for any
choice of the black hole angular momentum $a$. If the two orbits are required
to be frequency commensurable Keplerian orbits there is only one solution for
the two orbit radii and $a$. This model predicts that the microquasars have the
same $a$, and it predicts their black hole masses on the basis of the measured
HFQPOs in agreement with the dynamically determined masses. Application of this
model to the Galactic Center black hole Sgr A* using the recently measured QPOs
(Genzel et al., 2003, Aschenbach et al., 2004) leads to a black hole mass of
(3.28 +/- 0.13) x 10^6 M_odot, and the same $a$ as for the microquasars. The
possibility that all four sources have $a$=0.99616 suggests that this value is
the upper limit of $a$ imposed by general relativity. The same value for the
lower orbit radius and the same value for $a$ are also suggested by an analysis
of the general relativistic expression for the radial gradient of the orbital
velocity, which changes sign in a narrow annular region around the lower orbit
when $a$>0.9953.
