[frazwood]

I purchased this home 3-4 years ago... it already had a automated watering system for the lawn. The system has 7 zones for watering, the third one of which is no longer working.

I am assuming that I have a bad solenoid valve somewhere (does this seem likely?)... but I have no idea where it would be located so that I can replace it (I know almost nothing about how these systems are designed). Would there be a common place for all of the solenoids or would each solenoid be located near each zone?

[speedball1]

Would there be a common place for all of the solenoids or would each solenoid be located near each zone?

In my area sprinklers are a separate trade. However, we're exposed to them. Sprinkler solenoids are located under ground at the point that the sprinkler branch ties to the main.
Start checking at the control box. Zone #3 won't come on? Is the control box sending power to the solenoid? If it is then go out and track the branch back to the sprinkler main feed. You'll find your solenoid there. Good luck, Tom

[albinfla]

Usually, your zones will all merge in one or 2 valve boxes. You can use a voltmeter to test for 24 volts, by touching the white wire with one lead, and the colored wire on your solenoid with the other. In other words, there are 2 wires going to the solenoid coil on each solenoid valve. Take the wire nuts loose, then check them with your meter.

Do yourself a favor, and map out your zones. Then, label your solenoid valves with a paint pen or something similar. That will simplify your work next time.

You are correct, it is probably either the electric solenoid coil, or the diaphragm in the solenoid valve. If you can find a dealer who has the same brand as your valve, you can change the parts without having to change the whole valve.
Al