Which should the third paragraph of a business letter body do?

The third paragraph of a business letter is known as the 'call to action' paragraph.

The first paragraph states why you are writing; the second paragraph states the information necessary to accomplish the goal of the letter.

The third paragraph should tell the recipient what you expect them to do or the expected result of your letter. Always thank the recipient for their time and action. If the letter is just to inform (no result expected), then just thank the recipient for their time and attention.