  We show that three-body loss of trapped atoms leads to sub-Poissonian atom
number fluctuations. We prepare hundreds of dense ultracold ensembles in an
array of magnetic microtraps which undergo rapid three-body decay. The
shot-to-shot fluctuations of the number of atoms per trap are sub-Poissonian,
for ensembles comprising 50--300 atoms. The measured relative variance or Fano
factor $F=0.53\pm 0.22$ agrees very well with the prediction by an analytic
theory ($F=3/5$) and numerical calculations. These results will facilitate
studies of quantum information science with mesoscopic ensembles.
