disableButtons = false;

function loadArticle(id)
{
  if(disableButtons) return;
  switch(id){

    case 1 : url = "podstrona1.php";break;
    case 2 : url = "podstrona2.php";break;
    case 3 : url = "podstrona3.php";break;
    default : case 1;
  }
  disableButtons = false;
  startGETRequest(url, onComplete, onEnd);
}

function onComplete(text, xml)
{
  var dataDiv = document.getElementById('dataDiv');
  dataDiv.innerHTML = text;
}

function onEnd(text, xml)
{
  disableButtons = false;
}

<div id="selectorsDiv" class="mainDiv">
<input type="button" class="myButton" value="Wyślij wiadomość"
       onclick="loadArticle(1)" id="btn1">
<input type="button" class="myButton" value="Team Speak 3"
       onclick="loadArticle(3)" id="btn3">
<input type="button" class="myButton" value="Media społecznościowe"
       onclick="loadArticle(2)" id="btn2">
</div>