This is pretty straight forward, you must complete 3 Non-story missions by yourself (No co-op).
Acceptable side missions are:
Cult Outpost missions
Prepper Stash missions and finally
Any mission in your journal under the catergory: Side Missions.
Random world events such as killing a cult v.i.p., saving hostages, blowing up cult convoys, etc. Do NOT count towards this achievement.
My advice would be to liberate an outpost in any Holland Valley (John's Region). You will more than likely be able to receive a side mission from one of the civilians in the outpost. Once you complete that side mission, or before, it doesn't really matter. Take an rpg, several remote explosive or a helicopter with guns and go to the YES sign on the mountain north of US Auto on the left side of the map (Still in Holland Valley). Attack the sign and you'll receive a side mission to blow up the sign.
Depending on what order you did, you should get the achievement after completing the side mission from the outpost or blowing up the sign. If not, liberate another outpost and get another side mission. Do this until the achievement pops.