The Populous 3 Demo allows you to play a tutorial and 3 levels. Personally, i think you should just download the normal game, so you have 25 levels of increasing difficult that will get you ready for the online gaming, but if you are not sure weather or not you want to install the complete game, and need to see this demo first, go ahead.

 

Download it here.

 

Having only the Demo installed will not make you able to play online, so take that in mind.