  We present ESO/VLT FORS2 low resolution spectroscopy of red giant branch
stars in three massive, intermediate age ($\sim 1.7-2.3$ Gyr) star clusters in
the Large Magellanic Cloud. We measure CH and CN index bands at 4300A, and
3883A, as well as [C/Fe] and [N/Fe] abundance ratios for 24, 21 and 12 member
stars of NGC 1978, NGC 1651, NGC 1783, respectively. We find a significant
intrinsic spread in CN in NGC 1978 and NGC 1651, a signal of multiple stellar
populations (MPs) within the clusters. On the contrary, we report a null CN
spread in NGC 1783 within our measurement precision. For NGC 1978, we separated
the two populations in the CN distribution and we translated the CN spread into
an internal N variation $\Delta$[N/Fe]$=0.63\pm0.49$ dex. For NGC 1651 and NGC
1783, we put upper limits on the N abundance variations of $\Delta$[N/Fe]$\leq
0.2, 0.4$ dex, respectively. The spectroscopic analysis confirms previous
results from HST photometry, where NGC 1978 was found to host MPs in the form
of N spreads, while slightly younger clusters (e.g. NGC 1783, $<$ 2 Gyr old)
were not, within the limits of the uncertainties. It also confirms that
intermediate age massive clusters show lower N abundance variations with
respect to the ancient globular clusters, although this is in part due to the
effect of the first dredge up at these stellar masses, as recently reported in
the literature. We stress the importance of future studies to estimate the
initial N abundance variations, free of stellar evolutionary mixing processes,
by observing unevolved stars in young clusters.
