For 2 cells with times, just subtract, like =B1-A1.
To subtract 3 hours from the time in a cell, use this formula: =B2 - 3/24
NOTE: Use Calendar format 1904 if you need to display negative results; Tools/Options/Calculation and check the 1904 Date System checkbox. Note that 1904 calendar will offset all dates by 4 years 1 day, so be careful.