Mook, you don't say what kind of fence you're talking about either,
but if two rails warp, so will three.

I'm talking about standard privacy fence with the dogeared top.

http://images.lowes.com/product/054561/054561106084.jpg

One owed inward the other bowed outward ther other one sideways. Good lord.
these look terrible just afew months after installation. Probably cheap
wood.