  We present the photometric analysis of the external regions of three Galactic
Globular Clusters: NGC 6121, NGC 6397 and NGC 6752. The main goal is the
characterization of the multiple stellar populations along the main sequence
(MS) and the study of the radial trend of the different populations hosted by
the target clusters. The data have been collected using FORS2 mounted at the
ESO/VLT@UT1 telescope in UBVI filters. From these data sets we extracted
high-accuracy photometry and constructed color-magnitude diagrams. We exploit
appropriate combination of colors and magnitudes which are powerful tools to
identify multiple stellar populations, like B versus U-B and V versus
c_{U,B,I}=(U-B)-(B-I) CMDs. We confirm previous findings of a split MS in NGC
6752 and NGC 6121. Apart from the extreme case of omega Centauri, this is the
first detection of multiple MS from ground-based photometry. For NGC 6752 and
NGC 6121 we compare the number ratio of the blue MS to the red MS in the
cluster outskirts with the fraction of first and second generation stars
measured in the central regions. There is no evidence for significant radial
trend. The MS of NGC 6397 is consistent with a simple stellar population. We
propose that the lack of multiple sequences is due both to observational errors
and to the limited sensitivity of U,B,V,I photometry to multiple stellar
populations in metal-poor GCs. Finally, we compute the helium abundance for the
stellar populations hosted by NGC 6121 and NGC 6752, finding a mild (Delta Y ~
0.02) difference between stars in the two sequences.
