This chapter of Assassin's Creed: Odyssey walkthrough includes a description of all Ainigmata Ostraka hidden across Argolis. Below, you can find helpful information on locating these Ostraka: Two of Clubs, The Floor is Lava, and An Arm and a Leg.
The Floor is Lava
Location of Ainigmata Ostraka: Go to the enemy camp. The item can be found in front of the wicker baskets.
Riddle solution: Go to the Ancient Forge of Hephaistos, located inside a volcano. Remember that lava can kill you instantly. Jump between the rocks until you reach a rock with a human corpse on in. This is where you will find the item.
Two of Clubs
Location of Ainigmata Ostraka: Bronze Club of Periphetes. On the ground by the stairs.
Riddle Solution: Go to the camp called Hercules' Club. There, on top of the tree, you can find the item. You can kill all the enemies first, however, they will not stand in your way to this item.
An Arm and a Leg
Location of the Ainigmata Ostraka: Bronze Club of Periphetes. On the ground, by the stairs.
Riddle solution: Go to the Sanctuary. The item you are looking for can be found under the statue.