  To search for optical variability on a wide range of timescales, we have
carried out photometric monitoring of 3C 454.3, 3C 279 and S5 0716+714. CCD
magnitudes in B, V, R and I pass-bands were determined for $\sim$ 7000 new
optical observations from 114 nights made during 2011 - 2014, with an average
length of $\sim$ 4 h each, at seven optical telescopes. We measured multiband
optical flux and colour variations on diverse timescales. We also investigated
its spectral energy distribution using B, V, R, I, J and K pass-band data. We
discuss possible physical causes of the observed spectral variability.
