Last year Ubi was number two and Activision #3 in the US, IIRC. Nintendo was below both of them. Ubi doesn't service Japan so that's where the extra money is coming from.
Generally, fewer companies will breed an oligopoly, see the MPAA or RIAA for examples. Hell, we're already seeing a form of oligopolic practices, namely price fixing, in the videogame market. You don't see publishers trying to undercut each other, they all sell at 50$ because that's the accepted price. Sometimes they try to grab more with the most hyped games but generally games go for 50 unless they believe the game doesn't sell.