Dear Zeeshan,
Yes, we would normally apply the 3D (rotational augmentation) corrections before the Viterna extrapolation.
Regarding the aspect ratio (AR), please see the following forum topic: Extrpolation-S809-Viterna model-NREL Phase VI Turbine.
Regarding the max CD, AirfoilPrep recommends the equation max CD = 1.11 + 0.018*AR, which came from the following report (although I’m not familiar with the details): Viterna, L. A. and Janetzke, D. C., Theoretical and Experimental Power From Large Horizontal-Axis Wind Turbines. NASA TM-82944, September, 1982.
Best regards,