Actually, if you really want to talk about the 'power' of the drivetrain companies, it has long been in the following ranking:
The majority of sales for bicycles are either commuter/hybrid/comfort or mountain bikes.
Let's start off talking about the largest sales group, the commuter/hybrid/comfort range. Shimano has Tiagra and down from there... they've even got groups that don't have names, just number combinations. The specs for many bikes just read "Shimano Drivetrain" - and it's a mix. Not a great drivetrain for what we prefer, obviously, but the point I am making is that the sales are being made, and Shimano is right there. Now, SRAM? They're in this market too. Not nearly as prominent as Shimano, but they are there. Campagnolo? Who?
Now let's talk about the 2nd largest segment: mountain bikes. People tend to buy these for the same reason suburban housewives feel like they need an SUV truck: it looks durable, tough and has 'shocks' for "comfort". The majority of people buying mountain bikes may never see dirt in their lifetime. But hey, let's ignore that and just talk about all MTB in general - the serious riders and the posers. Drivetrains? It's either Shimano or it's SRAM. They've been competitive with each other for many, many years now and their groups are top-noche. Prior to SRAM there was SACHS in Europe (purchased by SRAM) - but anyway, SRAM has been there for a while.
....Then we've got Road bikes.
Shimano/Campagnolo Traditionally... SRAM comes into play lately and is part of the 'big 3'.
But, honestly? In the industry as a whole there were only a big '2' and those two were Shimano and SRAM. Campagnolo is long overdue for an evolution in how they operate if they want to have any success in the future... long overdue. If not, they'll be relegated to sales in a niche market with less and less ability to compete for innovation stemming from research, testing, design as the other companies continue to grow their market share and capital.