[devote]

<input id="test" type="text" />
<script>
    var elem = document.getElementById( 'test' ),
        oldFirstNumber = "";

    elem.onkeyup = function() {
        var num = this.value.replace( /[^\d]+/g, '' ).substr( 0, 1 );
        if ( num != oldFirstNumber ) {
            oldFirstNumber = num;
            if ( num == 7 ) {
                alert( "Ввели семерку" );
            } else if ( num == 3 ) {
                alert( "Ввели тройку" );
            }
        }
    }
</script>