Forty Thieves gets a third deck: twelve tableau piles, twelve foundations, and far more cards to sort before the stock runs dry.

Sixty Thieves Solitaire takes Thomas Warfield's Forty Thieves Solitaire and adds a third deck, spreading the cards across twelve tableau piles instead of ten, with twelve foundations instead of eight. It's a challenging game as a result, since three decks give you a lot more cards to sort through before the stock runs dry.
Marie Rose Solitaire is another solitaire game built the same way, if you'd like to compare.
If you run into anything odd or have an idea that would make the game better, please contact me.
Have fun playing!
There are twelve foundation piles.
Any ace in the foundation may be transferred to any vacant pile.
A card can be added to a foundation pile only if it's one rank higher and the same suit as the pile's current top card, so the only card that fits on a 6 of clubs is a 7 of clubs. There can be no more than 13 cards in a pile.
The top card of each foundation can be moved back into play if another pile will accept it.
There are twelve tableau piles of five cards each, all dealt face-up.
A card can be added to a tableau pile only if it's one rank lower and the same suit as the pile's current top card, so the only card that fits on a 10 of spades is a 9 of spades.
Any tableau card not covered by another card is free to be played onto the foundation or another tableau pile.
Only one card may be moved at a time; sequences aren't permitted.
Any card can fill an empty slot in the tableau.
There is one waste pile, and the remaining cards make up the stock.
Click the stock to deal one card to the waste. There's only one pass through the stock; no redeals.
The top card of the waste can be played to the tableau or foundation.