How do I format Excel to show timings.I want to show F1 one lap times to a thousandth of a second

Select the cells, press Ctrl+1, choose Custom, and enter:

[m]:ss.000

For a lap of 1 minute, 23.456 seconds, enter the value as:

0:01:23.456

Excel will display 1:23.456. Including the leading 0: makes it clear that you’re entering zero hours, one minute and 23.456 seconds.

If your lap times are already stored as seconds, such as 83.456 in A1, use =A1/86400 in another cell and apply the same format.

You need to do a custom format.  Go to your number format and then select "More Number Formats" then in the pop-up window you need to select 'Custom' on the left and then type in "mm:ss.000" in the right like shown below: