Who said this, where and why...

[Legolas]

The third was said by Elrond to the Fellowship right before they set off. (Beren87)

No.

The third is not Elrond. It was Legolas in Fangorn Forest, just before the appearance of Gandalf the White to the Three Hunters:

'We have journeyed a long way round,' said Legolas. 'We could have all come here safe together, if we had left the Great River on the second or third day and struck west. Few can foresee whither their road will lead them, till they come to its end.'

[ March 06, 2003: Message edited by: Legolas ]