The paper presents ten well-known developed stable flow regimes of an air/water/oil mixture in a horizontal pipe simulated numerically. The results have been tested by comparing the obtained fields of phase distribution and velocity with the results of experimental observations published in the literature. For sampling, four classical designs of isokinetic sampling probes guided by the existing standards for two-phase flows have been considered. The obtained results showed that the reliability of the estimation of the phase composition in the three-phase mixture by isokinetic sampling probes strongly depends on the flow regime. Therefore, in real oil production conditions with the presence of unstable and transient regimes, using the isokinetic probes without preliminary preparation of the flow in front of the sampling section is not recommended.