This method of eating is considered one of the best. When you eat with just one or two fingers, you don't take in enough food with each bite. This means you end up eating for a longer time to feel satisfied. Moreover, your digestive organs might not be happy with such small amounts of food per bite. They might have periods of inactivity while you're still eating! Eating only a few grains at once doesn't make the eating experience enjoyable or comfortable either.
On the other hand, eating with all five fingers can overwhelm your stomach with too much food in each bite. If one of your organs gets blocked with too much food, it could even lead to death. Plus, your body has to work extra hard to digest a large amount of food, making the whole process uncomfortable.
That's why using three fingers, following the example of the Prophet and those who follow him, is considered the best way to eat.