  We derive molecular-gas-phase $^{12}$C/$^{13}$C isotope ratios for the
central few 100 pc of the three nearby starburst galaxies NGC 253, NGC 1068,
and NGC 4945 making use of the $\lambda$ $\sim$ 3 mm $^{12}$CN and $^{13}$CN
$N$ = 1--0 lines in the ALMA Band 3. The $^{12}$C/$^{13}$C isotopic ratios
derived from the ratios of these lines range from 30 to 67 with an average of
41.6 $\pm$ 0.2 in NGC 253, from 24 to 62 with an average of 38.3 $\pm$ 0.4 in
NGC 1068, and from 6 to 44 with an average of 16.9 $\pm$ 0.3 in NGC 4945. The
highest $^{12}$C/$^{13}$C isotopic ratios are determined in some of the
outskirts of the nuclear regions of the three starburst galaxies. The lowest
ratios are associated with the northeastern and southwestern molecular peaks of
NGC 253, the northeastern and southwestern edge of the mapped region in NGC
1068, and the very center of NGC 4945. In case of NGC 1068, the measured ratios
suggest inflow from the outer part of NGC 1068 into the circum-nuclear disk
through both the halo and the bar. Low $^{12}$C/$^{13}$C isotopic ratios in the
central regions of these starburst galaxies indicate the presence of highly
processed material.
