Convert Seconds Into Hours, Minutes & Seconds

I have a spreadsheet that imports data relating to phone call durations. The information imports as a total number of seconds taken.

What I need to do is to change that number of seconds into hours, minutes and seconds.

Changing the format of the cells doesn't work.

I've tried everything I can think of, and run out of ideas.

example:

23 needs changing to 00:00:23
96 needs changing to 00:01:36
268 to 00:04:28
9374 to 02:35:14