There are three different Age Of Empires 3 scenario packs available on local download. Each pack has 100's of different scenarios to play.
Most of them are single-player but can be played multiplayer as well. Installation instructions are with the file description.

Age Of Empires 3: Scenario Pack #1 (145.46MB)

Age Of Empires 3: Scenario Pack #2 (109.85MB)

Age Of Empires 3: Scenario Pack #3 (49.12MB)