  The goal of this paper is to study possibilities of using first, second and
third massive stars in open clusters to estimate total cluster mass and
membership. We built estimator functions with the use of numerical simulations
and analytical approximations and studied the precision and error distribution
of the obtained estimator functions. We found that the distribution of the mass
of first, second and third massive stars shows strong power-law tails at the
high-mass end, thus it is better to use median or mode values instead of
average ones.
  We show that the third massive star is a much better estimator then the first
as it is more precise and less dependent on parameters such as maximum allowed
stellar mass.
