Summation Error in SSRS

From the image above, I want to sum the Time Spent column in my SSRS report. The issue is that this is not an int column but a varchar after finding the difference between two datetime columns. I want my result set to be in the format - 00:00:00 (form of hh:mm:ss). The sql query looks like: 

CONVERT(Varchar(10), Dateadd(Second, DATEDIFF(Second,StartDate,FinishedDate),0), 108)

 What do I have to do to get the right values back? Thank you.

Zionlite