Separate the seconds from the fractions of a second with a decimal point,
not another colon, then apply a custom number format of h:mm:ss.00 to the
cell.
And another FYI - you can enter fractions by typing 7 1/2 and Excel will
enter it in the cell as 7.5 but display it as 7 1/2. In your case I would
enter them as Jon said decimals.