Auto_TAB

............AutoTab.js------------------
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-- make this a separate .js file and add reference in head section of page.
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/* This script and many more are available free online at
The JavaScript Source!! http://javascript.internet.com
*/
var isNN = (navigator.appName.indexOf("Netscape") != -1);

function autoTab(input, len, e) {
    var keyCode = (isNN) ? e.which : e.keyCode;
    var filter = (isNN) ? [0, 8, 9] : [0, 8, 9, 16, 17, 18, 37, 38, 39, 40, 46];
    if (input.value.length >= len && !containsElement(filter, keyCode)) {
        input.value = input.value.slice(0, len);
        input.form[(getIndex(input) + 1) % input.form.length].focus();
    }

    function containsElement(arr, ele) {
        var found = false, index = 0;
        while (!found && index < arr.length)
            if (arr[index] == ele)
                found = true;
            else
                index++;
        return found;
    }

    function getIndex(input) {
        var index = -1, i = 0, found = false;
        while (i < input.form.length && index == -1)
            if (input.form[i] == input) index = i;
            else i++;
        return index;
    }
    return true;
}
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 <script type="text/javascript" src="JS/autoTab.js"></script>

     <asp:TextBox ID="txtph2" MaxLength="3"
         onKeyUp="return autoTab(this, 3, event);"   
         onkeypress="return isNumberKey(event)"
         runat="server"></asp:TextBox>