There is no single answer here - as the three words have different meanings MRS is pronounced missuz (with the stress on the i) and is used for married women MISS pronounced as written is used for an unmarried woman. MS is usually pronounced MUZ, and is a fairly new term - used only in the past thirty years or so - and was devised in conjunction with the women's Liberation movement to steer away from the need to classify a woman as married or not - as if it had a value.
Many women objected to having to state their marital condition, and so Ms was devised by using only the letters which occurred in both titles. The difficulty is in pronouncing it - and for many people older than thirty five they are still not comfortable about using this term, as it is not a common construction and has no clear precedent for pronunciation.
As for when to use each of these terms - I suggest you ask the recipient - "How would you like to be addressed?" and most women these days will give you there first name - so if you have to use the formal address you may need to persist.
Recieved a sympathy card from fiance's child, who has not spoken to me in months. Card was addressed as Ms. My first name was spelled wrong. Have been with her father 10 years. She is a teacher in her late 20's. The card was nice. Ignore envelope and bad taste? Or am I a Ms.
If I were in her shoes, I might use Ms too; as it's supposed to be a neutral term, it is possibly the easiest under the circumstances, though as a teacher and after ten years, she might have got the spelling right.