<form>

　　<input type="text" name="text1" maxlength="3" size="3"/>-

　　<input type="text" name="text2" maxlength="3" size="3"/>-

　　<input type="text" name="text3" maxlength="3" size="3"/>

</form>

function tabForward() {

    if(this.value.length == this.getAttribute('maxlength')) {

        for(let i = 0;i < this.form.elements.length;i ++) {

            if(this == this.form.elements[i]) {

                if(this.form.elements[i + 1]) {

                    this.form.elements[i + 1].focus();

                    return;

                }

            }

        }

    }

}

var form = document.forms[0];

var text1 = form.elements['text1'];

var text2 = form.elements['text2'];

var text3 = form.elements['text3'];

text1.addEventListener('keyup', tabForward, false);

text2.addEventListener('keyup', tabForward, false);

text3.addEventListener('keyup', tabForward, false);