  Fifty archival radio observations of the supergiant binary Cyg OB2 #5 using
the Very Large Array over 20 years are re-examined to determine the location
and character of the previously detected variable radio emission. The radio
emission from the system consists of a primary component that is associated
with the binary, and a non-thermal source (NE) that has been ascribed to a
wind-collision region (WCR) between the stellar winds of the binary and that of
a B-type star (Star D) to the NE. NE shows no evidence of variation in 23
epochs where it is resolved separately from the primary radio component,
demonstrating that the variable emission arises in the primary. Since NE is
non-variable, the radio flux from the primary can now be well determined for
the first time, most especially in observations that do not resolve both the
primary and NE components. The variable radio emission from the primary has a
period of 6.7+/-0.3 years which is described by a simple model of a non-thermal
source orbiting within the stellar wind envelope of the binary. Such a model
implies the presence of a third, unresolved stellar companion (Star C) orbiting
the 6.6-day binary with a period of 6.7 years. The variable non-thermal
emission arises from either a WCR between Star C and the binary system, or
possibly from Star C directly. The model gives a mass-loss rate of 3.4 x
10^{-5} solar mass/yr for Cyg OB2 #5, unusually high for an Of supergiant and
comparable to that of WR stars, and consistent with an unusually strong He I
1.083-micron emission line, also redolent of WR stars. An examination of radial
velocity observations suggests reflex motion of the binary due to Star C. The
natures of NE and Star D are also examined. (abridged)
