It was questionable since you asked how to set it with 1:3 as part of your question, so that's why I figured to clear it up. The threshold you should set accordingly that gives you good compression per ratio setting but not so much where you are pinning your gain reduction. From there it's tweaking and fine tuning by ear and comparing your results. Takes al ittle time.
As far as I know only NAT3 supports compressors. But I am not 100% on what features NAT2 will support if it does at all as far as compressors go.