如题虽然动作简单，但是好像实现起来很复杂

不太明白，123单独占一行？还是在第三行前文追加或后面追加？如
xxxx
xxxx
123
xxxx
还是
123xxx
或xxx123

rem 做个弟三行: 123xxxx例子
@echo off&setlocal enabledelayedexpansion
for /f "delims=" %%i in (test.txt) do (
set /a num+=1
if "!num!"=="3" ( echo 123%%i>>test1.txt ) else (
echo %%i>>test1.txt )
)
ren test.txt test.bak
ren test1.txt test.txt
echo 学习雷锋！
pause>nul

rem 123转行
@echo off&setlocal enabledelayedexpansion
for /f "delims=" %%i in (test.txt) do (
set /a num+=1
if "!num!"=="3" ( echo 123>>test1.txt&echo,%%i>>test1.txt ) else (
echo %%i>>test1.txt )
)
ren test.txt test.bak
ren test1.txt test.txt
echo 学习雷锋！
pause>nul

很晚了😴

@echooff&setlocal enabledelayedexpansion
(for /f "tokens=1* delims=:" %%a in ('findstr /n .* 1.txt') do (
set /a b+=1
if !b! equ 3 (echo,%%b%username%) else (echo,%%b)
))>
#&move /y #
1.txt

MOVE
+3

ECHO