3 Types of taxi

In GTA 4 there are 3 types of taxis: a minivan and two sedans. But only two of them can spawn in the game at the same time - a minivan and one of the sedans. Both sedans do not spawn at the same time. How to make sure that all 3 types of taxis are in traffic at the same time? I dug into the files "cargrp.dat", "vehicles.ide" and "handling.dat" - did not give the desired result.