title: multiline regex pattern that match 3 digits
body: <p>Can anyone please help me with a regex pattern that will match and check the following scenarios? I'm trying to match text that's separated with return characters. Each line should only have 3 digits (\d{3}) in it and up to 100 lines.</p>
  
  <p>This is ok:</p>
  
  <pre><code>123
  456
  789
  </code></pre>
  
  <p>This is not ok:</p>
  
  <pre><code>123
  123789
  456
  </code></pre>
  
  <p>This is not ok (note the empty line in the middle and at end):</p>
  
  <pre><code>123
  
  456
  </code></pre>
  
  <p>I would appreciate any suggestion and help. thanks.</p>