If you are writing a letter like this, you are going to need to make it clear to the boss why it would be a good idea (from her perspective) to let you work three days a week. I assume this is less than you are now working.
You will need to portray this as something that would be good for the business where you work. Perhaps you could say that you will be able to be a better employee if you do not have as much stress as you currently have.
You should probably also emphasize that you are positive about working where you do. You need to make it clear that you do not want to quit but that you need to have fewer hours so as to be able to A) be less stressed and B) continue to be able to work rather than having to quit.
if u heve a specific reason for working fo just 3 dayz u can give the reason to your boss....... but if u uaself dnt want to work then u should make out a good reason but i would suggest u to work because it will help u and the more u work the more u vl be experienced.......and your hardwork will pay 4sure.......:)