Explanation: you need to calculate the odds of doing it in any number of win/loss/draw combinations (ie. win every roll; or win the first roll, lose the next, win the rest; etc).
Fortunately a very nice website has already put up a program to do the work for you:
http://gamesbyemail.com/Games/Gambit/BattleOdds Looks like the odds are
44% if you attack the 3 last, and
50% if you attack it first, or anytime in the middle.
*edit* The reason for the big difference is that it's a lot easier to kill a single army at the end, because you're likely to be running out of your own armies, and rolls like 2 dice against 1 or 1 against 1 are a whole lot better than 2 or 1 against 2.