<?php

	$dir = 'D:\Activity\MyDoor2\temp\tmp'; // папка с ткст файлами
	$files = glob( $dir . '\*.txt' );
	
	foreach ( $files as $file )
	{
		$strings = file($file);
		
		$counter = 1;
		$s = '';
		foreach( $strings as $string )
		{
			if ( $counter == 3 )
			{
				$s .= str_replace(array("
", "\n"), array('', ''), $string) . '<!--more-->' . "
";
				$counter = 0;
			}
			else
			{
				$s .= $string;
			}
			$counter++;
		}
		file_put_contents( $file, $s );
	}

?>

<?php

	$dir = 'D:\Activity\MyDoor2\temp\tmp'; // папка � �к�� �айлами
	$files = glob( $dir . '\*.txt' );
	
	foreach ( $files as $file )
	{
		$strings = file($file);
		
		$counter = 1;
		$s = '';
		foreach( $strings as $string )
		{
			if ( $counter == 3 )
			{
				$s .= str_replace(array("
", "\n"), array('', ''), $string) . '<!--more-->' . "
";
				$counter = 0;
			}
			else
			{
				$s .= $string;
			}
			$counter++;
		}
		file_put_contents( $file, $s );
	}

?>

В конце каждой третей строки вставить текст

PHP