Although it's pretty hard to miss - just for those who may be reading ahead looking to grab this achievement on the way.
Normal or higher difficulty is required.
Mission 8: 'New Alexandria'.
Right at the very end of this mission, whilst flying a falcon you will have to defend and take out some anti-air guns. There are six of these, so there's plenty of opportunity to get the three needed. Just make sure if you're playing co op and a couple of you want the achievement that you don't destroy more than you need too, that way two of you can get it!
Edit: Comments have stated that there are apparently other turrets dotted around the mission, not just at the end. However, these are not what I nor my co op partner destroyed (or needed to) but if it makes it easier for anyone, it may be worth keeping an eye out for these too.