That is true now, because for historical reasons the Lib Dems managed to maintain that position of third party. It doesn't mean it would be that way if you actually had PR elections. UKIP and the BNP would see rather significant vote increases, and not all of it at the expense of the Tories. In addition, PR would put an end to the incumbency advantage that limits the losses of the big parties when they lose an election. If this election had been under PR, I dare say Labour would've gotten a fair few less votes than they got now, not merely fewer seats.

Of course, if your point is just about natural alliances, it's true that under PR, even with significant changes of the kind I described, the Tories would have a disadvantage in that the right-wing would be splintered in three parties, one of which is absolutely not viable for a coalition. But then, PR might also create changes within the parties, and perhaps force Labour and the Lib Dems to make the distinction between them a bit sharper, or even cause parts of parties to defect to others.