  We present the results of optical photometric observations of three extreme
TeV blazars, 1ES 0229$+$200, 1ES 0414$+$009, and 1ES 2344$+$514, taken with two
telescopes (1.3 m Devasthal Fast Optical Telescope, and 1.04 m Sampuranand
Telescope) in India and two (1.4 m Milankovi\'{c} telescope and 60 cm
Nedeljkovi\'{c} telescope) in Serbia during 2013--2019. We investigated their
flux and spectral variability on diverse timescales. We examined a total of 36
intraday $R-$band light curves of these blazars for flux variations using the
power-enhanced {\it F}-test and the nested ANOVA test. No significant intraday
variation was detected on 35 nights, and during the one positive detection the
amplitude of variability was only 2.26 per cent. On yearly timescales, all
three blazars showed clear flux variations in all optical wavebands. The
weighted mean optical spectral index ($\alpha_{BR}$), calculated using $B - R$
color indices, for 1ES 0229$+$200 was 2.09 $\pm$ 0.01. We also estimated the
weighted mean optical spectral indices of 0.67 $\pm$ 0.01 and 1.37 $\pm$ 0.01
for 1ES 0414$+$009, and 1ES 2344$+$514, respectively, by fitting a single
power-law ($F_{\nu} \propto \nu^{-\alpha}$) in their optical ({\it VRI})
spectral energy distributions. A bluer-when-brighter trend was only detected in
the blazar 1ES 0414$+$009. We briefly discuss different possible physical
mechanisms responsible for the observed flux and spectral changes in these
blazars on diverse timescales.
