a/c compressor connections

hello all,
i am trying to troubleshoot why my a/c compressor is not engaging on my '99 chevy k1500. i thought i knew what 2 of the 3 electrical connections on the compressor did what, but now i have my doubts.
before i go any further with the troubleshooting, i would like to get some clarification. can somebody tell me the purpose of the 3 connections?
one is at the front top of the compressor, near the serpentine belt. another is connected to the top of the compressor, going back toward the firewall, near where the freon hoses are connected. the third one is at the rear of the compressor.
thanks in advance...

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