  We present CCD photometry and proper motion studies of the three open star
clusters NGC 4103, NGC 5281, and NGC 4755 (kappa Cru). By fitting isochrones to
the colour magnitude diagrams, we found that all three objects are young open
star clusters with ages of at most t=45 Myr. They are located at distances from
approx. 1600 pc to 2200 pc, derived from distance moduli (m-M)_0 ranging from
11 mag to 12 mag. We combined membership determinations based on proper motions
and statistical field star subtraction to derive the initial mass function
(IMF) of the clusters. The shape of the IMFs could be represented by power laws
with exponents of Gamma=-1.46 +/- 0.22 for NGC 4103, Gamma=-1.60 +/- 0.50 for
NGC 5281, and Gamma=-1.68 +/- 0.14 for NGC 4755, when - as a reference -
Salpeter's (1955) value would be Gamma=-1.35. These results agree well with
other IMF studies of open star clusters.
