SIMBAD references

We present our optical photometric observations of three TeV blazars, PKS 1510-089, PG 1553+113, and Mrk 501, taken using two telescopes in India, one in Bulgaria, one in Greece, and one in Serbia during 2012-2014. These observations covered a total of 95 nights with a total of 202 B filter frames, 247 images in V band, 817 in R band while 229 images were taken in the I filter. This work is focused on multiband flux and colour variability studies of these blazars on diverse time-scales which are useful in understanding the emission mechanisms. We studied the variability characteristics of above three blazars and found all to be active over our entire observational campaigns. We also searched for any correlation between the brightness of the sources and their colour indices. During the times of variability, no significant evidence for the sources to display spectral changes correlated with magnitude was found on time-scales of a few months. We briefly discuss the possible physical mechanisms most likely responsible for the observed flux variability.