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.

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