Paragraph 1: This paragraph is an introduction to the letter and an introduction to you if you have not been in correspondence with this person before. If necessary, briefly introduce yourself. Go on to say what the reason is for your letter. Keep things concise and friendly.
Paragraph 2: This paragraph should expand on the topic brought up in the first paragraph. This is where all of your points should be raised. This paragraph can be as long as is needed depending on what has to be covered in the letter.
Paragraph 3: This paragraph is for concluding the letter. You should ask for any questions that you have asked to be answered, and you should sign off on a pleasant note, such as you are looking forward to hearing from them soon.