The rules are fairly simple. It's played on a 3x3 grid using the numbers 1 to 9. The numbers are arranged in a random order by the computer and then placed, hidden from the player, one in each square. At the start of the game, the computer reveals one of the numbers. You then have three turns, in each of which you can reveal the contents of another square. This gives you four numbers you can see and five you can't. You choose a line (horizontal, vertical, diagonal) and then the computer reveals all the numbers. The total of the three numbers in your line is used to award a prize.
The only prizes you really need to know are 7 8 9 (which gives 3,600 gold pieces) and 1 2 3 (which gives 10,000 gold pieces). The rest give smaller amounts such that it's always worth going for a 1 2 3 if you can and 7 8 9 if you can't.
Because the contents of the squares are determined at random, there usually isn't a 1 2 3 line present. If there is one, though, I can usually find it. I can say this with some confidence, because if there's one in all three of the day's games, this happens:
Sadly, these gold pieces are only good for items for sale from one vendor, most of which are rubbish. Still, I like Mini Cactpot so play it every day anyway.
It's more of a mechanic than a game, to be honest, but I don't care because I'm good at it.