Weekly Off When The Prefix Sufix Taken Casual Leave

one regular staff/officer taken 3 day cl ( 12/10 13/10 14/10/16) of prefix and 2 days of suffix on 16/10 and 17/10/16 is eligible to avail the weekly off on 15/10/16
by taking weekly off he passing the ot benefit to other employee

It is based on the company policy.
As per company regulation, you can have the policy that the weekly off or holidays can't be clubbed in the middle of CL leaves.
In your case, you can include the weekly off as CL days for calculation, if your company policy says so...

Generally, and as per service rules. period spent on casual leave is considered to be duty.

Please refer the leave policy of your company whether prefix and/or suffix is attached/calculated as leave or not. If your company does not have any leave policy please draft as soon as possible and implement it immediately before it is too late.

You have not provided any details of your company structure, size or other critical ,information. So getting a proper reply is difficult for you.
Weekly off is applicable only to someone who has worked for 6 days. So that is one important rule that helps you.
If you don't have your own HR and leave rules, them obviously you need refer to standing orders. In model standing orders the rule is clearly stated that holidays are not to be counted if prefixed and suffixed with leave. I think similar rule is also there for shop and establishment act