The origin of the ch sound is significant to deserve its own post, although it isn’t exactly a letter. What is it about c combined with h makes the “ch-” noise of leach or churn? And what makes it sound like k, as in choir or mechanic? And if mechanic has the k sound, why does the similar “machine” sound like sh?
I have a feeling this is going to be a confusing one.
Time to go back to Latin, where the ch sound was pronounced x. Once the language spread to France, the k sound (which was c because Classical Latin only uses k in Greek words) became “tsh,” or roughly what we would call a hard “ch”. It became a part of Old French, the begetter of English, and they started spelling the sound ch, as we know it today. After the Norman Conquest in 1066, the Anglo-Saxons started using ch in their words. As Middle English evolved, words that had been previously pronounced with a k started being pronounced with a ch (kinn became chin, carite-charity).
Meanwhile, Old French turned into Middle French, where the hard sound of ch was dropped completely, making it sh. Words continued to cross back and forth between the languages and in some words (like charade, from the eighteenth century or chastise from the thirteenth), the sh sound stuck.
As for the third pronunciation of ch, the hard k sound in character, chorus and the like stems from Greek. Like I said, Classical Latin uses k only in Greek words. The Latin for character is from the Greek word kharakter; the Latin speakers just switched the letter to c, which was a “true” letter of their language. And some places, English pronunciation followed suit.