As per the most reliable data available from UPSC Annual Report for Civil Services Examination 2011, Psychology (10.5%) had the best success-rate among these three optional subjects. Next, come Sociology (8.2%) and Public Administration (7.9%) is not far behind.
For Engineering graduations, none of these offer anything special that can help score well. It is always aptitude that matters the most while selecting the most suitable optional subject. If you have interest and understand the specific requirements each of these subject has, you can perform well with any of these optional subjects.
Yes, Sociology certainly requires good answer-writing skills and has capacity to reward candidates with good expression and language.