The results of numerical simulations presented in this paper are concerned with instability of a three member slender system subjected to Euler load. The investigated column is built up as a flat frame composed of three rods. In the internal one, the defect is present in form of a crack. The boundary problem has been formulated on the basis of a static criterion of instability. The boundary conditions associated with different types of supports are obtained by proper selection of parameters of the generalized load. On the basis of these results, the magnitude of bifurcation load can be determined.