What you need is a multi-channel transmitter/receiver in one of the unlicensed ISM (Industrial-Scientific-Medical) bands such as 315 or 433 MHz. In general the multi-channel ones seem to come in groups of four (4/8/12 etc.), so a 4-channel system will work for you, wasting one channel, or you could even use an 8-channel system, and not use the other 5 channels.
The cheapest route is to build your own; there are plans on the web for a couple of systems such as this one which uses a pair of encoder/decoder chips HT12E/HT12D available from Futurlec plus a transmitter and receiver.
However since you are a novice re electronics, DIY is probably not going to be a good route for you. This company, Blue Point Engineering, while not cheap, seem to have a number of modules that would be applicable to your situation. And more so, they offer custom engineering so they would be able to modify one of their transmitters to interface with the LED outputs of your PLC, and suggest a receiver which could directly drive the LEDs in the trucks. The receivers run off 12 volts, but check with them to make sure they can be hooked up directly to a truck's 12v power which is susceptible to transients which could kill the module. You may need a DC-DC converter like this one, which is specifically designed to protect against car or truck voltage variations and load dumps.
In any case, tell BPE what you want to do (your description in your question seemed clear enough) and they should be able to design a system for you.
Note that the transmitter will be sending to all of the receivers at the same time, so any truck in range of the transmitter (probably a few hundred feet at most), not just the one under the crane, will display the light indications. Transmitting only to the truck directly under the crane would be a lot more complicated.
Wouldn't it be easier to have the PLC outputs control three 110V floodlights (red, green, yellow) placed somewhere in front of the truck's path?