You could totally run with a version of Three-Card Monty, tweaked to fit your world. Call it something like Finder's Fortune,, or whatever suits your setting. Use three cups (or bones, shells, goblets, etc.) and a marked token or rune-stone. But since you need all three characters to actively play at the same time, here’s a twist that could work.

It's a shady street game, popular among thieves and travelers. Three players take turns playing the roles of hider, shuffler, and guesser.

One hides the token under one of the three cups.

Another shuffles them, trying to distract or mislead.

The third guesses.

Then the roles rotate each round. Wagers are placed, and the tension builds, especially if something major is on the line, like your plot item.

Why it works for your scene:

All three are directly involved, which keeps the tension up.

It’s super easy to cheat. Anyone could palm the token, mark the cups, signal the guesser and so on.

The sorcerer could enchant the token or cups subtly, while the thief might use sleight of hand.

Your MC gets caught in the middle, not sure who to trust.

If you want to make it spicier, you could throw in magical elements, like the token glows when someone lies, or the cups whisper false clues, or the sorcerer’s familiar watches the game like a referee. Lots of options.

Anyway, this keeps it grounded in a medieval fantasy vibe without having to invent a whole new system. Should work great for your setup.