Sounds like your 3rd zone may be using alot more water than the rest, causing a drop in pressure like that to cause the poppet to drop and leak. I would check zone 3 for leaks or ways to use less water with that zone.
Is zone 3 quite a bit different in terms of types of heads or numbers of heads on it?
Sprinkler Solutions, Inc.
Arizona and Colorado
www.sprinklersolutions.net
Sprinkler Solutions, Inc.
Arizona and Colorado
www.sprinklersolutions.net