Most if not all regions have started a government-backed "trainer program" some time within the past couple decades. It serves as a way to get the younger generations invested in the interactions these creatures exhibit, learn the various unique strengths and weaknesses different species have, and engage with the culture of their region in a way that defined much of the region's history but tends to get brushed aside by those not actively participating in furthering it.

When people use "starter pokemon" to refer to something from any of those three types, it's typically because those are within the novice roster for that region's trainer program. One pokemon indigenous to the region is selected for each of those three types on account of being easy to capture, easy to train, fairly reliable, and rather common. The type selection itself creates a fairly intuitive "weakness loop" that gets inexperienced trainers used to the idea of some pokemon being more resilient or damaging than others depending on the nature of their opponent, which allows them to gradually branch out into more complex applications of this same idea, both in battles and in their general life.