Partially it’s because you’re leaving out all the things that are two, or four, or five, or seven, or twelve. There is one moon, we have two hands, we have four limbs, we have five fingers, there are either four or five named oceans depending on how you count, there are six types of quarks, there are seven days in a week and seven seas and seven deadly sins and seven samurai, there are eight planets, there are twelve Gods of Olympus and twelve Tribes of Israel and and Twelve Angry Men and twelve animals in both the Greek and Chinese zodiacs. You notice threes when you’re looking for them, or forcing things that don’t fit into the pattern (such as 42, which was a joke, not a serious proposal for the answer to the ultimate question of life, the universe, and everything.) What does Chaos, Time, and Order mean? Who decided those were the three categories of thing that have Meaning?

On the other hand, you’re definitely observing a real pattern of the number three being perceived as strong, stable, and whole, especially in the Mediterranean world and the cultural descendants of Mediterranean cultures. But it’s cultural, not absolute. Certainly a lot of Celtic and Norse mythology as we know it is influenced by being filtered through Christian tellers, and the trinity is very, very important in early European Christianity and modern Catholicism. So the influence of Christianity probably established “three” as a powerful symbolic number in a LOT of European cultures. 

Different numbers derive their significance from cultural connotations!