function move(frmField,nextElem) {
    if(frmField.value.length == 3){
        document.text.elements[nextElem].focus();
    }
}


<input type='text' name='p1' id='p1' size='2' maxlength='3' onKeyUp="move(this,'p2');">
                                                     <input type='text' name='p2' id='p2' size='2' maxlength='3' onKeyUp="move(this,'p3');">
                                                     <input type='text' name='p3' id='p3' size='3' maxlength='4'>