If you want to play old maps such as Chernarus on Arma 3 you will need to use mods to do so. The best way is with the CUP series.
Simply download the mods and them to the root of your service, then add their names to the command line.
CUP Core: Needed before installing any other CUP mods, all others are optional.
https://steamcommunity.com/sharedfiles/filedetails/?id=583496184&searchtext=CUP
CUP Maps: Adds the maps such as Chernarus
https://steamcommunity.com/sharedfiles/filedetails/?id=583544987&searchtext=CUP
CUP Terrains:
https://steamcommunity.com/sharedfiles/filedetails/?id=853743366&searchtext=CUP
CUP Vehicles:
https://steamcommunity.com/sharedfiles/filedetails/?id=541888371&searchtext=CUP
CUP Weapons:
https://steamcommunity.com/sharedfiles/filedetails/?id=497660133&searchtext=CUP
CUP Units:
https://steamcommunity.com/sharedfiles/filedetails/?id=497661914&searchtext=CUP
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