put line2 in <h6></h6> and line3 in <h1></h1>

Try:

$result = preg_replace('#(^.+\n)(.+)<br/>(\r?\n)(.+)<br/>(\r?)#', '(^.+\n)(.+)<br/>(\r?\n)(.+)<br/>(\r?)', $original);

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What's the original state of this data?  Is it in a data base?  If your script somehow created the first snippet, it might be easier to just create the second snippet rather than creating the first and then transforming it into the second!

<?php
$original='
<h3>___________________________</h3>
Bob Adams<br/>
Bob real estate<br/>
<h3>___________________________</h3>
Susan<br/>
Susan real estate<br/>
<h3>___________________________</h3>
Mary Adams<br/>
Mary real estate<br/>
';

$result = preg_replace('#(^.+\n)(.+)<br/>(\r?\n)(.+)<br/>(\r?)#', '(^.+\n)(.+)<br/>(\r?\n)(.+)<br/>(\r?)', $original);
echo $result;

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'view source' output (almost same as input)

<h3>___________________________</h3>
Bob Adams<br/>
Bob real estate<br/>
<h3>___________________________</h3>
Susan<br/>
Susan real estate<br/>
<h3>___________________________</h3>
Mary Adams<br/>
Mary real estate<br/>

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What's the original state of this data?
html in this format
not in my database (yet)

Can you use a string in that way? I thought multi-line strings needed to be in a HEREDOC.

@kaufmed: Yes, you can define a string that way.  And yes, I agree with you that multi-line string definitions should be HEREDOC (or sometimes NOWDOC) because it makes for much easier punctuation, provides easy variables substitution, etc.

thanks

for a related question

line3 does not always contain the word 'real estate'

https://www.experts-exchange.com/questions/28176914/variation-to-pattern-of-line3.html