Normally, in the setup menu, you can turn each input on or off. You can usually even label the inputs based on what type of source you are plugging into them.
As you hit the input button on the TV or remote, it should switch through each input that is turned on. If it does not change several times (once for each press of the button), you likely have most (or all) of the unused inputs turned off. Just pick the input you want to plug the PS3 into (HDMI is best) and go into the settings to turn that input on. It will be labeled on the back, but you can probably rename it "game" if you want.
In the setup menu, look for "input selection" or "designate inputs" or "assign inputs" or something like that.