<form id="casmansForm">
  Name: <input type="name" id="userName" class="inputs"><br>
  Email: <input type="name" id="userName" class="inputs"><br>
  Text: <input type="name" id="userName" class="inputs"><br>
  <input type="submit" id="userSubmit" disabled><br>
</form>
<div id='alertMessage'></div>

var userName = document.getElementById('userName');
var userEmail = document.getElementById('userEmail');
var userText = document.getElementById('userText');
var userSubmit = document.getElementById('userSubmit');
var alertMessage = document.getElementById('alertMessage');

function checkForm(){
  if(userName.value == "" || userEmail.value == "" || userText.value == "")
{
    alertMessage.innerHTML = 'Please fill in form correctly';
    userSubmit.disabled = true;
    return false;
  } else {
    alertMessage.innerHTML = 'Thank you for filling in form';
    userSubmit.disabled = false;
    return true;
  }
}

userName.addEventListener("blur",checkForm,false);
userEmail.addEventListener("blur",checkForm,false);
userText.addEventListener("blur",checkForm,false);