[davem]

Originally Posted by Hilde

Is this simply a strange turn of phrase or is Gwaihir paying Gandalf back for a favor we are unaware of?

Its a common proverb, apparently:

"If you mean you think it is my job to go into the secret passages first, O Thorin Thrain'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. Perhaps I have begun to trust my luck more than I used to in the old days" he meant last spring before he left his own house, but it seemed centuries ago"but anyway I think I will go and have a peep at once and get it over. Now who is coming with me?" TH 'On the Doorstep'

Whether it was a specifically Hobbit proverb which Gandalf had picked up, or whether it was a commonplace saying in Middle-earth is another question. If the former we'd expect Gwaihir to respond to Gandalf with a 'Sorry, I don't follow.' Seems we have to go with the latter, which implies a collection of common sayings known by members of many different races. Where did these sayings originate? And why, exactly, does the third time pay for all?

EDIT

Maybe these sayings were part of a collection of lore passed down even among Hobbits:

Pippin was silent again for a while. He heard Gandalf singing softly to himself, murmuring brief snatches of rhyme in many tongues, as the miles ran under them. At last the wizard passed into a song of which the hobbit caught the words: a few lines came clear to his ears through the rushing of the wind:
Tall ships and tall kings
Three times three,
What brought them from the foundered land
Over the flowing sea?
Seven stars and seven stones
And one white tree.
"What are you saying, Gandalf?" asked Pippin.
"I was just running over some of the Rhymes of Lore in my mind," answered the wizard. "Hobbits, I suppose, have forgotten them, even those that they ever knew."
'No, not all," said Pippin. 'And we have many of our own, which wouldn't interest you, perhaps. 'Minas Tirith'

EDIT again:

Hobbits are not quite like ordinary people; and after all if their holes are nice cheery places and properly aired, quite different from the tunnels of the goblins, still they are more used to tunnelling than we are, and they do not easily lose their sense of direction undergroundnot when their heads have recovered from being bumped. Also they can move very quietly, and hide easily, and recover wonderfully from falls and bruises, and they have a fund of wisdom and wise sayings that men have mostly never heard or have forgotten long ago.

Shippey in a talk at Birmingham claims he counted over 70 proverbs in LotR. Gandalf's words to Frodo are interesting:

'I can't believe that Gollum was connected with hobbits, however distantly,' said Frodo with some heat. 'What an abominable notion!'
'It is true all the same,' replied Gandalf. 'About their origins, at any rate, I know more than hobbits do themselves. And even Bilbo's story suggests the kinship. There was a great deal in the background of their minds and memories that was very similar. They understood one another remarkably well, very much better than a hobbit would understand, say, a Dwarf, or an Orc, or even an Elf. Think of the riddles they both knew, for one thing.'

These common riddles are obviously very ancient & come from before the settling of the Shire. What's interesting is the way they have survived down to Bilbo's time. Shippey pointed out that Gollum's riddles have their roots in Anglo-Saxon riddles (as found in the Exeter Book for example) while Bilbo's seem to be recent creations (ie invented by Tolkien) so the process of inventing riddles seems to have gone on. Bilbo therefore has an advantage over Gollum - he knows the old ones but has access to new ones whereas Gollum only has the ones he learnt 500 years previously. So we seem to have ancient lore (& riddles) handed down & new ones invented - as with Bilbo's 'Out of the frying pan, into the fire' & 'Never laugh at live dragons.' So we get a collection of lore building up over the years, some of it the common property of all races, some of it unique to each race (as with Butterbur's: 'But there's no accounting for East and West as we say in Bree' & Pippin's ironic response "'Handsome is as handsome does' as we say in the Shire)