In SP, you build three warriors to INCREASE THE CHANCES OF WORKER-STEALING. After all, your first two warriors might get killed by an animal, or wander off in the wrong direction. Works reasonably on epic. Works poorly on Normal. Once again, settings matter. Worker-stealing won't work at all on MP (or it shouldn't). Once again, SETTINGS MATTER. But the reason to build 3 warriors on MP is to rush/choke. Humans are smarter than AI, BUT they start with less. So the objectives, depending on whether you're playing AI or human, are different.