I am currently trying to set up rains barrels in two locations for two different projects. I am only going to ask about one for now since the other one will be for veggies and wont be ready until spring.

I need to know how to set up these 3 15 gal barrels as back up rain barrels. Basically I want the main rain barrel to fill the other three. I have all the pvc parts I need but I am a bit lost as to how high the back up barrels need to be in comparison to the main one. How do I set these up to return water to the hose?