Thanks for the reply, the PSU is a corsair http://www.newegg.com/Product/Product.aspx?Item=N82E16817139007
When I have 3 x 6970 in the rig, the system will start for 1/2 a second and then dies, nothing would power on (though mobo still show the small power indicator light). If I put 2 x 6970, then system would boot up fine.
A 1000w psu should be more than sufficient but we need to know what psu you are using. I do not recommend undervolting any cpu below its specs. Regardless if it seems stable, long term damage still applies. Also please be more specific on what you mean by "rig won't start". Do the fans power up on the cards? Does anything power up at all? We need more info.