If the number you have represents the number of seconds and you want to format it to hh:mm:ss format then use the following formula:
=FormatNumber(Floor(<number of seconds>/3600) ,"00") & ":" &
FormatNumber(Floor(Mod(<number of seconds> ,3600)/60) ,"00") & ":" &
FormatNumber(Mod(Mod(<number of seconds> ,3600) ,60) ,"00")