Three card combos are not that interesting (since they do not come up very often), but this one is nice in that it is actually two different combos that just happen to combo nicely together.
First:
Baron+Crossroads is nice combo
Baron works well with lots of Estate density (and is able to increase that density itself if there isn't an Estate to discard). Crossroads works great with higher green density too. The extra actions from Crossroads means you can also over-invest in Barons. If you can consistently play more than one Baron a turn you should be able to wrap up the game quickly.
Second:
Baron+Farmland
Baron+Estate gives you +$4. A couple of coppers and you now have enough for a Farmland. You can use Farmland to upgrade Estates into Barons, Coppers into Estates and Farmlands into Provinces. You should be able to avoid buying Golds entirely.
Then you put them together, like in this game:
http://dominion.isotropic.org/gamelog/201205/06/game-20120506-130517-23fdc2c9.htmlThe only purchases I made were Barons, Crossroads, Farmlands and Provinces. It was over in 14 turns on piles, but I could very easily have run down the entire Province pile (More likely three of the four piles - Estates, Crossroads, Barons or Farmlands - would have run out on their own without a concerted effort to end the game on anyone's part).
Open Crossroads/Baron. Play Crossroads first. Discard Estates with Barons when you can, otherwise, gain Estates. Buy Farmlands or Provinces when you get a double Baron-discard.
Fast and fun.
Counters:
Discard Attacks: Discard attacks hurt it a lot - you would be forced to choose between Baron-Estate or Crossroads-Estate-Estate and hope for the best.
Cursers: Curses likely make it much more difficult to line things up. Usually it doesn't matter what you draw with tis deck (Crossroads lets you use your green. Barons let you discard your Estates. If you don't have Estates, Crossroads lets you play multiple Barons, Barons let you gain more Estates, etc.). But curses make a mess of things. Mountebank is especially bad as the extra coppers reduce the deck effectiveness a lot.
Comments welcome.