You need to change your text field to a timestamp field. You change how the field is displayed from the inspector.
Did you confirm those are summary fields and not calculation fields
The format is HH:MM:SS and in the calculation it is a text output
That won't work. As Stacy indicated you need a different result type for this calculation, number, time and timestamp are all possible and then calculations that use this field's value will work.
Note that time and timestamp fields store their values as a number in seconds. A value in a time field is the time since midnight in seconds. A value in a TimeStamp field is the number of seconds since Midnight of 1/1/0001 to the date, time shown.
So if you subtract two time or two timestamp fields, you get a value in seconds. You can chose a number result type and divide by 3600 seconds to get a value in hours or you can select either a time or timestamp result type. If the elapsed time exceeds 24 hours, you need to use a timestamp to accurately display the results.
Thank You All for the responses. I changed the calculation output to a number and added /3600 to the calculation.