  We present and analyze the variability of three Seyfert galaxies on intra-day
timescales. We have analyzed in a uniform manner the 38 longest ( $>30$ ks)
observations made for NGC 4051, MCG$-06-30-15$ and NGC 4151 by XMM-Newton
between 2000 and 2015. The nuclei were quite active during essentially all of
these observations and the overall X-ray fluxes (0.3-10 keV) varied by an order
of magnitude. Most of the observations do appear to show characteristic
timescales, estimated through their auto-correlation functions, ranging between
$\sim 2.9$ ks and $\sim 45.3$ ks. The hard (2-10 keV) and soft (0.3-02 keV)
bands are very well correlated but consideration of their hardness ratios show
that the sources typically soften during flares. We also provide new estimates
of the central black hole masses for these three AGN that support the
hypothesis that Narrow Line Seyfert 1 galaxies have relatively small central
black holes.
