There are a total of 3 Datacrons on Ord Mantell, each very easy to get.
Codex Entry - Galactic History 12: The Birth of the Republic
Coordinates: -655, -574
The first Datacron is on Savrip Island, the top of the hill. The Datacron is easy to get, you need to walk onto the northwest path of the hill (see where the char is on the below map), you need to get to the right..follow the rocky steps, until you reach it.
Codex Entry - Galactic History 13: The First Great Schism
Coordinates: -976, 207
This Datacron is located below the entrance to the Seperatist Stronghold (western beach - main island).
Red Matrix Shard
Codex Entry - Galactic History 14: The Fall of Tion
Coordinates: 770, 134
The last Datacron is located in a small hole into the ground, north beach of Mannett Point island. The Datacron is guarded by an elite, very easy to solo. It's very easy to spot so further instructions are useless here.