window.onload = showOtherTownField;

function showOtherTownField (){
    var town = document.getElementById('id_town');
    var other = document.getElementById('id_other_town');
    if (town && other) {
        if (town.options[town.selectedIndex].value == 3) {
            other.style.display='inline';
            // other.parentNode.parentNode.childNodes[0].innerHTML='Название';
        }
        else {
            // other.parentNode.parentNode.childNodes[0].innerHTML='';
            other.style.display='none';
        }
    }
}