I, for one, am ready for the reveal...Pretty Please & Thank You. 
I'm quite familiar with the 1st two riddles because I happen to live with a bona fide Tolkien expert (He thinks he is, at any rate) but I'm completely stumped on the last one. I don't want to resort to googling because that would bend my moral code but I'm close to losing that battle of wills...
Good job not Googling.
I think I know number three. I do know number one, but not two. 
I can't remember now if you sent me your guesses, but you could have.
I finally managed to puzzle my way through the third, though in a manner entirely unexpected to me, and the first two were not much trouble for me. If you aren't being sent guesses anymore, it may well be time to reveal the answers.
All right everyone, my apologies, I fully intended to post the answers hours upon hours ago but the internet went out without warning or explanation and I kept testing and testing and testing and it kept not working, so I gave up. Again, my apologies for making you all wait, it was not my intention.
Riddle #1
This thing all things devours:
Birds, trees, beasts, flowers;
Gnaws iron, bites steel;
Grinds hard stones to meal;
Slays king, ruins town,
And beats high mountain down.
Answer: Time
Riddle #2
It cannot be seen, cannot be felt
Cannot be heard, cannot be smelt.
It lies behind stars and under hills,
And empty holes it fills.
It comes first and follows after,
Ends life, kills laughter.
Answer: Darkness
Riddle #3
What does man love more than life,
fear more then death or mortal strife,
What the poor have the rich require,
and all contented men desire.
What misers spend and spendthrifts save
and all men carry to the grave?
Answer: Nothing
Congratulations to Wiry, azokka361, Gwendy, and Audren for correctly answering all three riddles!
PrincessMimi and Vinyl get partial credit for correctly guessing some!
