@echo off
setlocal enabledelayedexpansion
set js=1
for /f "delims=" %%i in (a.txt) do (
    set a[!js!]=%%i
    set a[!js!]=%a[!js!]:----=-%
    echo %a[!js!]%
    set /a js+=1
)
cls
set jslast=js
set js=1
set jst=0
:loop
set /a jst+=3
set /a o=jst-1
set /a t=jst-2
echo %a!t!% %a!o!% %a!jst!%>>b.txt
echo %a!t!% %a!o!% %a!jst!%
set /a js+=1
if %js% leq %jslast% goto loop
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@echo off
setlocal enabledelayedexpansion
set js=1
for /f "delims=" %%i in (a.txt) do (
    set a[!js!]=%%i
    call set a[!js!]=%%a[!js!]:----=-%%
    call echo %%a[!js!]%%
    set /a js+=1
)


cls
set jslast=%js%
set js=1
set jst=0
:loop

set /a jst+=3
set /a o=jst-1
set /a t=jst-2
call echo %%a[!t!]%% %%a[!o!]%% %%a[!jst!]%%>>b.txt

call echo %%a[!t!]%% %%a[!o!]%% %%a[!jst!]%%

echo %jst% %jslast%
if %jst% LEQ %jslast% goto loop
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<# :
@echo off
powershell -NoProfile -ExecutionPolicy bypass "Get-Content -literal '%~f0' |Out-String|Invoke-Expression"
pause
#>
$txt=gc .\a.txt|%{$_.replace("----","-")}
0..(($txt.Length-$txt.Length%3)/3)|%{$txt[3*$_]+" "+$txt[3*$_+1]+" "+$txt[3*$_+2]>>b.txt}
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@echo off
(for /f "tokens=1,2 delims=-" %%a in ('type "1.txt"') do (
    set /a n+=1
    set /a m=n%%3
    set /p=%%a-%%b <nul
    set /a test=1/m 2>nul || echo,
))>"2.txt"
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假设楼主的源文件中不包含!^等特殊字符，批处理代码如下...

@echo off &setlocal enabledelayedexpansion
(for /f "delims=" %%a in (1.txt) do (
	set "_s=%%a"&set "s=!s!!_s:----=-!"&set/a "n+=1"
	if !n! equ 3 (echo,!s!&set "n=0"&set "s=")
))>2.txt
pause&exit/b
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若源文件中包含!^等特殊字符，批处理代码如下（有幸set/a变量能在循环体内自由出入变量延迟期 ）...

@echo off &setlocal
(for /f "delims=" %%a in (1.txt) do (
	set "_s=%%a"&set/a "n+=1"
	setlocal enabledelayedexpansion
	set/p="!_s:----=-!"<nul
	if !n! equ 3 (echo,&set/a "n=0")
	endlocal
))>2.txt
exit/b
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如果批处理代码中采用 set/p... 输出字符串，要求不能在头部包含空格和=等字符，否则空格会被删除，=会导致报错退出。因此为避免入坑则要改用如下代码...
call可用于set和echo但不能用于if语块，故还须有劳‘延迟变量!n!和!s!’来串场解围

@echo off &setlocal
(for /f "delims=" %%a in (1.txt) do (
	set "_s=%%a"&set/a "n+=1"
	call set "s=%%s%%%%_s:----=-%%"
	setlocal enabledelayedexpansion
	if !n! equ 3 (
		echo,!s!
		endlocal
		set/a "n=0"&set "s="
	) else endlocal
))>2.txt
pause&exit/b
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