I'd put in my vote for St. Petersburg, Spain, Munich. It's hard to envision any empire that holds all three of these and does not possess every stalemate line.

Vienna, Budapest, Trieste. Insta-win.

Hecks: I'd guess that a majority of powers holding St. Petersburg, Spain, and Munich in a game with experienced players do not win.

To try and give a more useful answer to the question:

There is going to be no set of three centers such that controlling two of the three gives you a win, because there are several stalemate lines splitting the board into two halves, and one half will have at most one of the three centers. But, that's also not really the question you're asking, I think -- what you're really asking is what the territories are that dominate their region, essentially squares that you should be looking to get control of. Not surprisingly, most of these are not supply centers -- that's a key feature of the board design.

With that said, there are a few vital territories, holding disproportionate influence and lying in critical positions where either they're the key to holding or to breaking stalemate lines. Off the top of my head, the three most critical for fleets and for armies:

For fleets:

1. NTH: You cannot hold the Isles without it, and its occupier usually holds the balance of power in the Low Countries, and often in Scandinavia. Borders a whopping six centers, and it often takes twice as many units to defend against an opponent holding NTH as your opponent needs to keep it.

2. ION: Only borders three centers, but it's easy to hold from both sides, and whoever holds it will eventually control most of Italy.

3. MAO: Not as critical as it looks at first, but the most important bottleneck, both to hold and to breach.

For armies:

1. Galicia. The complex geography of the eastern board is really its own topic, but a good rule of thumb is that whomever controls Galicia is the dominant land power in Eastern Europe. This is a bit counterintuitive in terms of the geography (at first glance, Ukraine and Budapest look more important, and probably Serbia as well), and takes bit of working out to see exactly why.

2. Gascony: The key to France and its natural five centers. France can usually survive a German A Bur - Mar or A Bur - Par, but Bur - Gas is usually fatal if not immediately destroyed.

3. Munich: Can be held from most directions, and lies along many stalemate lines. Thus, grabbing Munich before the other side is a key feature of most endgames.

I agree with everything right above, with emphasis on Gascony. If France holds Gascony well, he'll be hard to crack, and if someone else gets to Gascony, France is a dead man.