Was looking around and saw this elsewhere on TA.com:
TP Industries Arms Race:
"Once you complete enough of Trevors story you will gain the ability to purchase properties, and McKenzie Field Hangar was the property that I was outside when this happened so I suspect you will be in a similar position! If not, McKenzie Field Hangar will be marked on your map as a Property for purchase so just set a waypoint and head there.
You will need $150,000 to buy the field and once you have done so, there's a cutscene where you meet the Control Tower operator Carlos.
Carlos will now inform you that there is an ongoing arms trade 'war' going on, so you will be required to either use the dune buggy provided to carry out weapons drop collections (travel to a set location, sometimes racing against rival gunrunners and sometimes staying offroad to avoid the police) or using the plane provided to either drop weapons or bombs on customers (or rivals). Once you have completed 5 of each type of mission, the achievement will unlock.
Each time you complete a plane or dunebuggy mission, you may need to drive away from the area for a short while so the next job(s) can 'spawn' but it's possible to do 5 of each without having to do any side missions or main missions in between."
I guess for the last part of that, you could also just quicksave.
EDIT: Also, since 'Three Man Army' is stated as missable, you may want to throw it up in the Roadmap under the Missable Achievements.