Good morning Mr. Kilmer, I am working on a 2002 Chevrolet Malibu, trying to add freon to the AC system but I cannot get the compressor to engage, I found the switch with 3 wires but do not know which ones to cross to engage compressor, can you help?

Was the system opened up and now you need to charge it? If so, be sure to put a vacuum on it for a while, and then start charging it. If you don't have a vacuum pump to do it, just have a shop do it for you as if you try to charge it without putting a vacuum on it, it won't cool properly and could have compressor failure if there's moisture in there.

the system was not opened, the a/c was working but not getting cold. then compressor stopped coming on.

I see, the next step would be to verify refrigerant charge with a gauge set. Then if that shows to be ok, then will need to determine why the compressor is not running, either shorted compressor clutch field coil, bad wiring, or an a/c control issue like a blown fuse, bad pressure sensor, hvac controller issue, bad relay, or an ecu issue.

Putting battery power directly to the compressor (or jumping the compressor relay) would be a quick way to determine if it's got a bad field coil, or it has another issue.

can this be done at the wiring at the switch that is on the a/c line behind the radiator?

The line switches are used by the PCM to determine whether to allow the compressor to kick on (if a low pressure switch) or forcibly turns it off (if a high pressure switch). Be careful when messing around with sensor wiring, trying to "hotwire" an issue at the sensor can lead to PCM damage if you don't know what you're doing. Also trying to force the compressor to run if low or out of refrigerant can damage it as well.

with the car off is it ok to unplug the compressor wires and send power to see if the magnets engage

Sure, that's the best way to determine if the field coil is the issue. Also could do that when the engine is running to see if it then cools or not, but only do it for a very short time, as that may have a variable displacement compressor which will overheat if it's not being controlled by the pcm.

thanks and goodbye