We summarize results of the analysis of the optical variability of the continuum and emission-line fluxes in the 3C 390.3 nucleus during 1992–2014 . The [ O iii ] \lambda 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 [ O i ] \lambda 6300 narrow line are completely different from that of [ O iii ] . The reverberation lags are found to be 88.6 \pm 8.4 , 161 \pm 15 , and 113 \pm 14 d 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 ) \times 10 ^ { 9 } M _ { \odot } and ( 2.81 \pm 0.38 ) \times 10 ^ { 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 3C 390.3 nucleus after removing a long-term trend . This trend has been expressed by a slowly varying scale factor 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 3C 390.3 nucleus is an ‘ outsider ’ in the relationship between optical luminosity and black hole mass . Its Eddington ratio is E _ { bol } / E _ { Edd } = 0.0037 .