  The BL Lac object 3C 371 has been observed by the Transiting Exoplanet Survey
Satellite (\textit{TESS}) for approximately a year, between July 2019 and July
2020, with an unmatched 2-minute observing cadence. In parallel, the Whole
Earth Blazar Telescope (WEBT) Collaboration organized an extensive observing
campaign, providing three years of continuous optical monitoring between 2018
and 2020. These datasets allow for a thorough investigation of the variability
of the source. The goal of this study is evaluating the optical variability of
3C 371. Taking advantage of the remarkable cadence of \textit{TESS} data, we
aim to characterize the intra-day variability (IDV) displayed by the source and
identify its shortest variability timescale. With this estimate, constraints on
the size of the emitting region and black hole mass can be calculated.
Moreover, WEBT data is used to investigate long-term variability (LTV),
including understanding spectral behaviour of the source and the polarization
variability. Based on the derived characteristics, information on the origin of
the variability on different timescales is extracted. We evaluated the
variability applying the variability amplitude tool that quantifies how
variable the emission is. Moreover, we employed common tools like ANOVA
(ANalysis Of VAariance) tests, wavelet and power spectral density (PSD)
analyses to characterize the shortest variability timescales present in the
emission and the underlying noise affecting the data. Short- and long-term
colour behaviours have been evaluated to understand the spectral behaviour of
the source. The polarized emission was analyzed, studying its variability and
possible rotation patterns of the electric vector position angle (EVPA). Flux
distributions of IDV and LTV were also studied with the aim of linking the flux
variations to turbulent and/or accretion disc related processes.
