Anita, The M&S will retro on a 3 wire system if you bought the M&S DMC3-4 model. M&S designed the DMC3-4 to work on 3 or 4 wire systems. If you have a different model it's a possibility to make it work but you will lose some features. If you have further questions you can e-mail me or call me at 877-435-0670 and I would be glad to help. Robert Pinell IntraSonic Technology www.intrasonictechnology.com

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