Sincronizar vídeos (play um, pause outros)

video.html

<video width="426" height="240" id="v1">
  <source src="video1.mp4" type="video/mp4">
  <source src="video1.webm" type="video/webm">
</video>

<video width="426" height="240" id="v2">
  <source src="video2.mp4" type="video/mp4">
  <source src="video2.webm" type="video/webm">
</video>

<video width="426" height="240" id="v3">
  <source src="video3.mp4" type="video/mp4">
  <source src="video3.webm" type="video/webm">
</video>

<script>
var v1 = document.getElementById("v1");
var v2 = document.getElementById("v2");
var v3 = document.getElementById("v3");

v1.onplay = function () { v2.pause(); v3.pause(); };
v2.onplay = function () { v1.pause(); v3.pause(); };
v3.onplay = function () { v1.pause(); v2.pause(); };
<script>