<body>
<div id="mainDiv" class="mainDiv">
<div style="width:50%;margin:0 auto;text-align:center;">
  <input type="button" value="Pierwsza tre" class="myButton" id="btn1"
         onclick="showDialog(1);" style="margin-top:10px;" />
</div>
<div style="width:50%;margin:0 auto;text-align:center;">
  <input type="button" value="Druga tre" class="myButton" id="btn2"
         onclick="showDialog(2);" style="margin-top:10px;" />
</div>
<div style="width:50%;margin:0 auto;text-align:center;">
  <input type="button" value="Trzecia tre" class="myButton" id="btn3"
         onclick="showDialog(3);" style="margin-top:10px;" />
</div>
</div>
</body>