In A3, I have a formula of the form =A1/A2/24, where A2 is time (date serial
number) formatted h:mm:ss. (A1 is a number formatted as General.)

When A3 is formatted as General, Excel changes the format to h:mm:ss every
time I edit A3. I have to change the format back to General manually, which
is a nuisance.

I can avoid this by formatting A3 as Number. But I would like it remain
General.

Is there any option setting that disables this autoformat heuristic?

I don't mind if it turns off all "intelligent" autoformat selection.

I am using MS Office Excel 2003 SP3.