this.createEmptyMovieClip("my_triangle_mc", 1);
my_triangle_mc._x = 75;
my_triangle_mc._y = 75;

drawTriangle(my_triangle_mc, 100, 4, 0x999999,100, 0x3366FF, 100);

function drawTriangle(target_mc:MovieClip, sideLength:Number, lineHeight:Number, lineColor:Number, lineAlpha:Number, fillColor:Number, fillAlpha:Number):Void {
var tHeight:Number = sideLength * Math.sqrt(3) / 2;
with (target_mc) {
beginFill(fillColor, fillAlpha);
lineStyle(lineHeight, lineColor, lineAlpha);
moveTo(sideLength / 2, 0);
lineTo(sideLength, tHeight);
lineTo(0, tHeight);
lineTo(sideLength / 2, 0);
endFill();
}
}