Please help me write a letter asking my boss if I can work three days a week.

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.