grep -v -x -F -f file2.txt file1.txt >file3.txt

findstr /x /v /i /l /g:file2.txt file1.txt >file3.txt

@echo off
setlocal disableDelayedExpansion

::Define the files
set "file1=test1.txt"
set "file2=test2.txt"
set "file3=test3.txt"

::Create an LF variable containing a line feed character
set LF=^


::The above 2 blank lines are critical - do not remove

::Create a modified version of file2 that escapes any backslash
::EOL is set to a linefeed so that all non blank lines are preserved
::Delayed expansion is toggled on and off to protect ! characters
>"%file2%.mod" (
  for /f usebackq^ eol^=^%LF%%LF%^ delims^= %%A in ("%file2%") do (
    set "ln=%%A"
    setlocal enableDelayedExpansion
    echo(!ln:=\!
    endlocal
  )
)

::Find lines in file1 that are missing from file2.mod
findstr /vixlg:"%file2%.mod" "%file1%" >"%file3%"

::Delete the temporary file2.mod
del "%file2%.mod"

@echo off
setlocal disableDelayedExpansion

::Define the files
set "file1=test2.txt"
set "file2=test.txt"
set "file3=test3.txt"

::Create an LF variable containing a line feed character
set LF=^


::The above 2 blank lines are critical - do not remove

::Find lines in file1 that are missing from file2.mod
::EOL is set to a linefeed character so that all non blank lines are preserved
>"%file3%" (
  for /f usebackq^ eol^=^%LF%%LF%^ delims^= %%A in ("%file1%") do (
    set "found="
    for /f usebackq^ eol^=^%LF%%LF%^ delims^= %%B in ("%file2%") do (
      if %%A==%%B set found=1
    )
    if not defined found echo %%A
  )
)