I would use a Potentiometer and play with it, monitor the voltage. I think you should stay in the range of 0.9v-1.1v at first, just to be safe. I'm use to play with the S1 board but never tried with the S3. the S1 had 4 chains of chips, so you needed 4 Pot / resistance per board.

There is all the clock and timing to be found with the voltage you want to use, that is the hardest part.



Found this ...

 Typical hash rate and power
Voltage(V) HashRate(GH/s) Current(A) Total power(W)  W/GH
0.72          14.18                10.40          7.49                0.528
0.75          15.75                12.24          9.18                0.583
0.80          17.33                15.20          12.16              0.702
0.85          17.33                20.00          17.00              0.981

EDIT: Check that => https://bitcointalk.org/index.php?topic=771979.60

EDIT2: It looks like the Resistance to be change is R17 ... and it's 8 per board not 4 like the S1