  The observed nuclear X-ray emission in the radio-quiet category of active
galactic nuclei (AGN) is believed to be from a compact region, the corona
situated in the vicinity of the central supermassive black holes (SMBH). The
shape of the X-ray continuum, among other factors, depends on the temperature
of the corona ($\rm{kT_{e}}$). The launch of the Nuclear Spectroscopic
Telescope Array (NuSTAR) has led to the determination of the high energy
cut-off ($\rm{E_{cut}}$; and thereby $\rm{kT_{e}}$) in many AGN. In a handful
of sources, multiple observations with NuSTAR have also revealed changes in
$\rm{E_{cut}}$. In this work we aimed to investigate the variation in
$\rm{kT_{e}}$ in three AGN, namely NGC 3227, NGC 5548 and MR 2251$-$178 using
more than one epoch of data on a source from NuSTAR. We carried out spectral
analysis of multiple epochs of data acquired using NuSTAR on the three sources
including a few new observations not published so far. By fitting
Comptonization model to the data we determined the temperature of the corona
and also investigated changes in $\rm{kT_{e}}$ if any in these sources. In NGC
3227, we found evidence for variation in $\rm{kT_{e}}$. We found no correlation
of $\rm{kT_{e}}$, photon index ($\Gamma$), reflection fraction ($R$) and
optical depth ($\tau$) with flux, while, $\tau$ is found to anti-correlate with
$\rm{kT_{e}}$. These could be due to more than one physical process at work in
the source that causes the change in $\rm{kT_{e}}$. Conclusive evidence for the
variation in $\rm{kT_{e}}$ is not found in MR 2251$-$178 and NGC 5548.
