Re: Batch modification of source clips.

For the sake of anyone else in this dilemma, in Excel I would take the end media field, and create 3 empty columns to the right.

Then select the media end column, and use "text to columns", with colon being the delimiter.

If you have drop frame timecode, you may have to repeat this step on the resulting seconds;frames column, using semi-colon as delimiter.

Now make a formula that takes 3 seconds from the seconds column. Of course, if that column value is less than 3, you'll have to "borrow" from the minutes column, and maybe borrow from hours

Formula to subtract seconds in new seconds column:
=if(secondsCelladdress <3, (secondsCelladdress+60)-3, secondsCelladdress - 3)

Formula to borrow from minutes in new minutes column
=if(secondsCelladdress <3, if(minutesCelladdress =0, (minutesCelladdress + 60) - 1, minutesCelladdress - 1), minutesCelladdress)

Formula to borrow from hours in new hours column
=if(secondsCelladdress <3, if(minutesCelladdress = 0, hoursCelladdress-1), hoursCelladdress), hoursCelladdress)

or something along those lines.