I don't think the first three Ratchet games, including Tools of Destruction had a difficulty option to pick through. I know there is a challenge mode for every Ratchet game out there, but I was thinking more of a difficulty option once you actually start playing the game.
The only games so far that has this kind of option are Deadlocked, Quest For Booty and A Crack in Time.
But moving on…
In Deadlocked, I think I've played it on Medium difficulty at first.
A Crack in Time on the other hand was first played through Medium, but I tried the game in Hard and Hardcore after finishing the game for the first time.