update text of textbox on check of checkbox

<input type="checkbox" id="one" onclick="updatebox()"/>1
<input type="checkbox" id="two" onclick="updatebox()"/>2
<input type="checkbox" id="three" onclick="updatebox()"/>3
<input type="text" id="tbtb"/>

I have 3 checkboxes and a textbox. I want that when all checkboxes are checked the textbox value would be like "123" and if one of them is not checked the value should go like "12" or "13" or "23". I dont need comma for this, I just need these to use as conditional values. To be specific here I want that they would still be in order though they are not checked consecutively in order. I need the java script function for this

and also when they will be unchecked the textbox should not contain the value they have. like when all are checked then u uncheck #1 the value should be "23". "1" should be removed on the text. coz right now the code i have that:

var one = document.getelementbyID('one');
var textbox = document.getelementbyID('tbtb').value;
if (one.checked)
{
textbox = textbox + "1";
}

so what's happening here is everytime a checkbox is checked and unchecked simultaneously the value of textbox keep adding another "1" it goes like "2311111". PLEASE HELP :)