There are 3 Datacrons on Tython, each very easy to get.
Codex Entry - Galactic History 9: The Force Wars
This Datacron is located in a cave, near the Jedi Temple.
Codex Entry - Galactic History 10: The Tion Cluster
Coordinates: -641, -70
This Datacron is located near the Ruins of Kaleth. You get into the ruins, you'll notice a stone column that's collapsed over a wall (left side as you enter). Once you get on top, keep it straight, until you cross-over another collapsed stone column (which connects to a hill). Turn on your right and you'lll be able to see the Datacron.
Blue Matrix Shard
Codex Entry - Galactic History 11: The Tionese Face the Hutt Empire
Coordinates: -92, 920
The last Datacron is located within the Forge Remnants - make a right and follow the path until you reach some ruins, fall down, you reach a small camp with a tent (there are 2 mobs there) - notice the ruins on the right of the camp - there's the Datacron (you need to jump on the rocky ledge).