i can't be fucked figuring the whole thing out right now and i have shit to do today, but here's some psuedocode (which won't work but will give you an idea of what to do)
//set your variables first
var startermoves:Array = new array;
var middlemoves:Array = new array;
var finishermoves:Array = new array;
var randNum:int = new integer;
var combo:String = "";
//shove in as many moves as you want here
startermoves = ["face punch", "spinning kick", "uppercut", "etc"]
middlemoves = ["low punch", "middle punch", "high punch", "punch flurry", "low kick," "axe kick", "etc]
finishermoves = ["flying kick", "face slam", "piledriver", "roundhouse kick", "special ability", etc]
//whenever you want the combo to be generated (ie. on a button click or frame event or whatever):
randNum = Math.round(Math.random() * startermoves.length);
combo += startermoves(randNum); //add the data for the randomly generated array position to the combo string
//this middle moves could be repeated several times to generate longer combos
randNum = Math.round(Math.random() * middlemoves.length);
combo += middlemoves(randNum);
randNum = Math.round(Math.random() * finishermoves.length);
combo += finishermoves(randNum);
//assuming "comboTxt" is a dynamic text field on stage
comboTxt.text = combo;
or some shit like that