  We have carried out single and multi-site photometry of the three Beta Cephei
stars Beta and 15 CMa as well as KZ Mus. For the two stars in CMa, we obtained
270 h of measurement in the Stromgren uvy and Johnson V filters, while 150 h of
time-resolved Stromgren uvy photometry was acquired for KZ Mus. All three stars
are multi-periodic variables, with three (Beta CMa) and four (15 CMa, KZ Mus)
independent pulsation modes. Two of the mode frequencies of 15 CMa are new
discoveries and one of the known modes showed amplitude variations over the
last 33 years. Taken together, this explains the star's diverse behaviour
reported in the literature fully.
  Mode identification by means of the amplitude ratios in the different
passbands suggests one radial mode for each star. In addition, Beta CMa has a
dominant l=2 mode while its third mode is nonradial with unknown l. The
nonradial modes of 15 CMa, which are l <= 3, form an almost equally split
triplet that, if physical, would imply that we see the star under an
inclination angle larger than 55 degrees. The strongest nonradial mode of KZ
Mus is l=2, followed by the radial mode and a dipole mode. Its weakest known
mode is nonradial with unknown l, confirming previous mode identifications for
the star's pulsations.
  The phased light curve for the strongest mode of 15 CMa has a descending
branch steeper than the rising branch. A stillstand phenomenon during the rise
to maximum light is indicated. Given the low photometric amplitude of this
nonradial mode this is at first sight surprising, but it can be explained by
the mode's aspect angle.
