It's a good question this one. Presume you have the number of seconds
to convert in cell N13 then put this formula in cell N14

=INT(N13/3600)&":"&INT(((N13/3600)-INT(N13/3600))*60)&":"&INT((((N13/3600)-INT(N13/3600))*60-INT(((N13/3600)-INT(N13/3600))*60))*60)

I imagine this could be done more succinctly, I'd be interested to see
other answers.

HTH