To capture and enemy building send your foreman over to the enemy building of your choice. Hover over it with the formant selected and your icon will turn tor for green flashing arrows.
Press
to being destroying the fence. Once completely destroyed the property is yours. The AI however will more than likely send one of their own builders or foreman, or multiples of each to stop you. If you have moved your builders over to the property with your formant, you can use these to attack any interference.
Once you have captured three buildings the achievement will pop. You do not need to capture 3 buildings in one game as this is cumulative over all games played.