  We report the discovery of a third kilohertz quasi-periodic oscillation (kHz
QPO) in the power spectra of the low-mass X-ray binaries 4U 1608-52 (6.3
sigma), 4U 1728-34 (6.0 sigma), and 4U 1636-53 (3.7 sigma) which is present
simultaneously with the previously-known kHz QPO pair. The new kHz QPO is found
at a frequency that is 52.8+-0.9 Hz, 64+-2 Hz, 58.4+-1.9 Hz higher than the
frequency of the lower kHz QPO in 4U 1608-52, 4U 1728-34, and 4U 1636-53,
respectively. The difference between the frequency of the new kHz QPO and the
lower kHz QPO increased in 4U 1608-52 from 49.6+-1.4 Hz to 53.9+-0.5 Hz when
the frequency of the lower kHz QPO increased from 672 Hz to 806 Hz.
Simultaneously the difference between the frequency of the new kHz QPO and the
upper kHz QPO increased by ~60 Hz, suggesting that the new kHz QPO is unrelated
to the upper kHz QPO. In 4U 1636-53 a fourth, weaker, kHz QPO is simultaneously
detected (3 sigma) at the same frequency separation below the lower kHz QPO,
suggesting the new kHz QPOs are sidebands to the lower kHz QPO. We discuss the
nature of this new kHz QPO and its implications on the models for the kHz QPOs.
