  We present aluminium, magnesium, and silicon abundances in the metal-poor
globular cluster NGC 6752 for a sample of more than 130 red giants with
homogeneous oxygen and sodium abundances. We find that [Al/Fe] shows a spread
of about 1.4 dex among giants in NGC 6752 and is anticorrelated with [Mg/Fe]
and [O/Fe] and correlated with [Na/Fe] and [Si/Fe]. These relations are not
continuous in nature, but the distribution of stars is clearly clustered around
three distinct Al values, low, intermediate, and high. These three groups
nicely correspond to the three distinct sequences previously detected using
Stromgren photometry along the red giant branch. These two independent findings
strongly indicate the existence of three distinct stellar populations in NGC
6752. Comparing the abundances of O and Mg, we find that the population with
intermediate chemical abundances cannot originate from material with the same
composition of the most O- and Mg-poor population, diluted by material with
that of the most O- and Mg-rich one. This calls for different polluters.
