Prova cosi:
on (release) {
premuto = "a";
gotoAndStop (30);
}
on (release) {
premuto = "b";
gotoAndStop (30);
}
on (release) {
premuto = "c";
gotoAndStop (30);
}

e poi al frame 30:
if (premuto == "a") {
loadMovieNum ("a.swf", 1);
} else if (premuto == "b") {
loadMovieNum ("b.swf", 1);
} else if (premuto == "c") {
loadMovieNum ("c.swf", 1);
}