Modders frequently use existing DLC slots to add custom vehicles, weapons, or clothing to the single-player mode. While many modern mods use a custom "addon" folder (like patchday28ng or custom-named dlcpacks), older modding tutorials often instructed players to replace files directly inside older archives like mpchristmas3 to replace standard game assets with custom ones.
: If you use FiveM, ensure your CitizenFX.ini file points correctly to your main GTA V installation folder. update x64 dlcpacks mpchristmas3 dlc.rpf