  We have studied the rapid X-ray variability of three extragalactic TeV
gamma-ray sources; Mrk 421, Mrk 501 and PKS 2155-304. Analyzing the X-ray light
curves obtained from ASCA and/or RXTE observations between 1993 and 1998, we
have investigated the variability in the time domain from 10^3 to 10^8 sec. For
all three sources, both the power spectrum density (PSD) and the structure
function (SF) show a roll-over with a time-scale of the order of 1 day or
longer, which may be interpreted as the typical time-scale of successive flare
events. Although the exact shape of turn-over is not well constrained and the
low-frequency (long time-scale) behavior is still unclear, the high-frequency
(short time-scale) behavior is clearly resolved. We found that, on time-scales
shorter than 1 day, there is only small power in the variability, as indicated
by a steep power spectrum density of f^{-2~-3}. This is very different from
other types of mass-accreting black-hole systems for which the short time-scale
variability is well characterized by a fractal, flickering-noise PSD
(f^{-1~-2}). The steep PSD index and the characteristic time-scale of flares
imply that the X-ray emitting site in the jet is of limited spatial extent; D >
10^{17} cm distant from the base of the jet, which corresponds to > 100
Schwarzschild radii for 10^{7-10} M_solar black-hole systems.
