how do i sum a range of durations in the format HH:MM:SS

you should be able to format the cells as a duration (since this format is ambiguous whether it is time or duration. then use the function SUM().

assuming the column with the durations is B, then you would sum like this:

=sum(B)

My preferred format is the "other" format:

the full list of parts of the duration are shown in the screenshot just below "Units". you an enter a "full" duration like this:

"1w 2d 3h 4m 5s 6ms"

that is One week, two days, three hours four minutes five seconds 5 milliseconds

you can enter only the parts you need to enter