Spectral variability of the 3C 390.3 nucleus for more than twenty years.
  I. Variability of the broad and narrow emission-line fluxes
  We summarize results of the analysis of the optical variability of the
continuum and emission-line fluxes in the 3C390.3 nucleus during 1992-2014. The
[OIII]5007 flux increases monotonically by $\approx$30 per cent in 2003-2014.
The narrow Balmer lines show similar monotonic increase, while the variability
patterns of the [OI]6300 narrow line are completely different from that of
[OIII]. The reverberation lags are found to be 88.6$\pm$8.4, 161$\pm$15, and
113$\pm$14d for the H$\beta$, H$\alpha$, and H$\gamma$ broad emission-lines,
respectively. The reverberation mass of the central black hole equals to
(1.87$\pm$0.26)$\times10^9\,M_\odot$ and (2.81$\pm$0.38)$\times10^9\,M_\odot$,
for the H$\beta$ and H$\alpha$ lines and assuming a scaling factor that
converts the virial product to a mass to be f=5.5. A difference between both
masses can point to a difference between kinematics of the H$\alpha$ and
H$\beta$ emission regions. We show that the reverberation mapping can only be
applied to the entire period of observations of the 3C390.3 nucleus after
removing a long-term trend. This trend has been expressed by a slowly varying
scalefactor c(t) in the power-law relationship between the line and continuum
fluxes: $F_{line}\propto c(t)\,F_{cont}^a$. We find the power-law index $a$
equals to 0.77 and 0.54 for the H$\beta$ and H$\alpha$ lines, respectively. The
observed relationship between the Balmer decrement and the optical continuum
flux is as follows: $F(H\alpha)/F(H\beta) \propto F_{cont}^{-0.20}$ and
$F(H\beta)/F(H\gamma) \propto F_{cont}^{-0.18}$. The 3C390.3 nucleus is an
"outsider" in the relationship between optical luminosity and black hole mass.
Its Eddington ratio is $E_{bol}/E_{Edd}$ = 0.0037.
