What three things must be included on every piece of mail before sending it?

Two items are essential : the correct postage and a correct mailing address, including the addressee's name/address/city/state or country/zip or postal code. The third, the return address, is vital if either of the other two is wrong.

A fourth item, which may determine postage or deliverability, is the class of mail for anything that is not obviously a letter. This is important for the proper handling of ALL items sent internationally, and can affect speed of delivery domestically.

In the US, stamped mail over 13 ounces must be presented at a retail counter.