The English owner's manual for the part number listed is available at...
Page six (6) indicates the back panel of your Plasma TV has two HDMI inputs, one component (RGB) and a specialized AV input/output connector. The latter requires a cable that is popular in Europe. Adapters to use it with US standard S-Video and RCA composite video are difficult to locate.
HDMI and component cables are available at big box stores (Best Buy, WalMart etch) but are much cheaper from generic internet sellers.
I am not sure about the second half of your question concerning "get to channel 3 to get to all other channels).
I must assume you have told the TV to "scan" for other channels (cable or over the air (OTA) with the black antenna/RF cable connected directly to the back of the TV.
If you have done so I am guessing you have connected the antenna cable first to a RECORDER or CABLE BOX. In many cases, these will only output channel 3 or 4 and block all other channels when turned on. In order to see other channels, you will need to buy a 2-way RF splitter so that both the recorder or Cable box can be connected at the antenna cable at the same time. Please note, doing so will mean you cannot see the output of the recorder or cable box on channel 3 or 4 but must use its the video/audio (HDMI, component, composite, S-Video) outputs from it to the TV. Again, this splitter is available from big box stores or the internet.