  We present the results of photometric observations of three TeV blazars, 3C
66A, S5 0954+658 and BL Lacertae, during the period 2013--2017. Our extensive
observations were performed in a total of 360 nights which produced $\sim$6820
image frames in BVRI bands. We study flux and spectral variability of these
blazars on these lengthy timescales. We also examine the optical Spectral
Energy Distributions of these blazars, which are crucial in understanding the
emission mechanism of long-term variability in blazars. All three TeV blazars
exhibited strong flux variability during our observations. The colour
variations are mildly chromatic on long timescales for two of them. The nature
of the long-term variability of 3C 66A and S5 0954+658 is consistent with a
model of a non-thermal variable component that has a continuous injection of
relativistic electrons with power law distributions around 4.3 and 4.6,
respectively. However, the long-term flux and colour variability of BL Lac
suggests that these can arise from modest changes in velocities or viewing
angle toward the emission region, leading to variations in the Doppler boosting
of the radiation by a factor ~1.2 over the period of these observations.
