To do this you have to fight your way through these keeps and raise your flag at the keep usually by the last enemies.
The 3 keeps are locations are.
The Western Approach - It is Located to the far east of the map. Go North-West from Camp Craggy Ridge and the entrance is under the keep that you gain entrance by going up a well.
Emprise Du Lion - It is on the South-West part of the map. It is on your way during a quest on this map about templars mining red lyrium and after you finish a boss fight it is just up the stairs after the fight. Located North-West of Sahrnia Quarry and West of the Tower Camp.
Crestwood - It is located is the West part of the map and is also part of a quest line that if you follow you will be able to find. It is south of the village of Crestwood and West of the Three Trout Farm Camp.