In general, I never looked at ST3 as the final goal of a multistage. Maybe usually the most active stage of most multistagers.
ST4 not only has the combo of past 3 stages (minus healing in most cases) but ST4 also has extra scripting not found in any other stages except ST4.
The last stage is a massive subliminal by itself. Hence when we manifest the goals of ST1, ST2 and ST3, it makes it easier to run ST4 and get results for the 4th stage faster with less reconciliation compared to jumping to ST4 in the first go.
Plus we can run any of the stages depending on our goals too. Some people might love Khan Black ST3 if they want to focus on cultivating their sexual energy for a longer period of time. That focus won’t be there with ST4.