How do we get an hour, minute and second difference between two datetimes?
CURRENT_TIMESTAMP returns the current datetime something like 2017-03-09 15:19:53.770. 

CONVERT(VARCHAR(10), CreatedDate, 108) returns a string with only time 15:19:53.

The second parameter of DATEDIFF takes a datetime so your string is implicitly converted to a datetime. In lack of something better to use for a date SQL Server uses 1900-01-01 so your second parameter ends up to be 1900-01-01 15:19:53.000 and the function returns the number of minutes since the beginning of the previous century.

Remove the convert in the second parameter and it should work just fine for you.