Puoi accederci con l'indice, la prima form ha indice 0, quindi ci accedi con document.forms[0].
Prova questo esempio:
codice:
<script>
  function Prova() {
    alert(document.forms[0].input1.value);
    alert(document.forms[1].input1.value);
    alert(document.forms[2].input1.value);
  }
</script>
<body onLoad="Prova()">
<form>
  <input type="text" name="input1" value="input1form0">
  <input type="text" name="input2" value="input2form0">
  <input type="text" name="input3" value="input3form0">
</form>
<form>
  <input type="text" name="input1" value="input1form1">
  <input type="text" name="input2" value="input2form1">
  <input type="text" name="input3" value="input3form1">
</form>
<form>
  <input type="text" name="input1" value="input1form2">
  <input type="text" name="input2" value="input2form2">
  <input type="text" name="input3" value="input3form2">
</form>
</body>