Thrice shall pay for... what?

[Inziladun]

Originally Posted by Ibrîniðilpathânezel

I'll grant that this does sound like a Hobbit bit of phraseology. Even so, Bilbo and Sam don't bother to mention the other previous instances that are being counted as a part of the third time; Gandalf does. It makes me think that something particular was in his mind.

Not to belabour the point, but Bilbo said it another time, before the previous quote I mentioned.

"If you mean you think it is my job to go into the secret passage first, O Thorin Thráin's son Oakenshield, may your beard grow ever longer," he said crossly, "say so at once and have done! I might refuse. I have got you out of two messes already, which were hardly in the original bargain, so that I am, I think, already owed some reward. But 'third time pays for all' as my father used to say, and somehow I don't think I shall refuse."

It's not proof by any means, but the recurrence of the phrase throughout the books pretty well convinces me it was just a throwaway remark on Gandalf's part, possibly having the dual meaning of a subtle 'Thank you and Good Night' to Gwaihir.