I Think a simple solution will work,
1) right click on cell in excel file and chose format cell, set format to number.
2) load the script with the below condition.
IF( Field3='' OR NOT ISNUM( Field3),0, Field3) AS Field
FROM your sheet,...
NOT ISNUM(Filed3), will check weather Field3 has number data or string, string like #N/A, so it will be replaced with 0
Hope it helps....