Wow I read the thread a second before I was about to post a solution identical to bolt's lol.
But yeah that's how you do it. I call it a failsafe trigger. They're not as pretty as cleanly keeping track of all possabilities, but as long as you're not mangling up the order of your other triggers they do their job just fine.
Abstract Concept++
Ok, I skipped most of the posts, but, I'm sure there's a way to make this work.
Instead of having it be, bring 1 this unit, 1 this unit, 1 this unit, create this unit...
have it add a certain score amount for every unit you bring.
Then, if after 3 units, your score matches that of a Hero, you get the hero.
Otherwise, you get IT.
Of course, you'd have to choose numbers such that they can't overlap, so you get the wrong hero for your combination when you should have got nothing...
which becomes MUCH harder depending on how many units you have, 3 is best...
Also, I'm pretty sure you could make an even easier and quicker method by using more than 1 player scores, move unit triggers to keep track of units, and so forth.