Learn as much as you can about ur company's IT infrastructure. How are the networks connected? Where are the switches and routers? Where are the servers? Do you even have access to any of the servers? This is kind of a good test of whether the company has faith in you.

Also find out from ur colleagues what are the normal problems end users face and how to solve it ASAP. In a non-IT environment, solving an IT problem in a few seconds is of paramount importance.

Basically the best way is to equip urself with enough knowledge that you can handle the workload if in some weird event all ur 3 colleagues are not around.