Reliability for summation index from binary variables

Hello all.

This might be simple to some of you but I'm stuck with it at the moment. Hence this call for help.

I'm trying to create a sum index call "social media use" from three binary variables: use social networking sites, use blogs and use video hosting sites (all coded 0=No and 1=Yes).

Would be very grateful if you could show me how to assess the reliability of this index. Many thanks in anticipation.