Spectral variability of the 3C 390.3 nucleus for more than 20 years --
  II. Variability of the broad emission-line profiles and He II $\lambda4686$A
  emission-line fluxes
  Results of the analysis of the variability of the H$\beta$ and H$\alpha$
broad emission-line profiles and the He II $\lambda4686$A emission-line fluxes
in the 3C 390.3 nucleus during 1992-2014 are present. The observed
velocity-dependent lag for the Balmer lines is similar to that expected from
the Keplerian disc configuration, although there are some differences.
Probably, a radial infall motion can be present in the broad-line region of 3C
390.3 in addition to the Keplerian rotation. The lag of the broad He II line is
$26\pm 8$d, significantly less than that of the Balmer lines, so the He II
emission region is much smaller in size. In terms of the power-law relationship
between line and optical continuum fluxes with slowly varying scalefactor
$c(t)$: $F_{line}\propto c(t)\,F_{cont}^a$, the power $a$ is $1.03$ for the
broad He II line, while according to Paper I the power is equal to $0.77$ and
$0.54$ for the broad H$\beta$ and H$\alpha$ lines, respectively. It means that
the variability amplitude is the largest in the He II, less in H$\beta$, and
more less in H$\alpha$. However, the Balmer lines contain a long-term trend
that is not seen in the helium line. The narrow He II line is variable with the
amplitude (max-to-min ratio) $R_{max}\approx 3$ that is much greater than the
variability amplitudes of both the narrow Balmer lines and the narrow [O III]
$\lambda$5007A line.
