The mixing behavior of three gas streams was investigated numerically by computational fluid dynamics (CFD) for 16 different geometries to gain insight for the construction of soot measuring systems. The overall goal was to find the design that leads to the fastest mixing of all incoming gas components for a given pipe length by numerical simulations. For this purpose, a main pipe with two symmetrically arranged side inlet pipes was considered, where the angle of inclination of the side pipes and the inflow conditions were varied. Upon the change of the angle of inclination, a transition from a conform to a counter flow is observed. As a variant of the simulation setup, the junction of the three pipes was enclosed by a spherical mixing chamber. The dependency on the angle is much less pronounced in the presence of the additional spherical chamber, which, however, in most cases results in a slower mixing of the gas streams. We found, in general, that the required pipe length to reach a sufficiently homogeneous gas mixture decreases with increasing inclination angles exhibiting the best performance at obtuse angles.