I have the same, problem, most of DLC stuff is unlocked by taking first outpost(Aka turtorial with Dennys) and when you liberate first watch tower, there are two end-game gestures at the end of the multiplayer match, you have them unlocked instatly at the beggening of the game(you dont have to start story/co-op/multiplayer) so if you dont have this(and this DLC content is fastest to check-comparing to starting over story and finishing full proluge wich take 1 hour), you still didn't have any of DLC content IN-GAME, i loooked at every post on FarCry3 in Ubisoft Forums, you are the first one who posted this here, doesn't mean you are the only one, i have same problem, but not so many pepole know about this offical forum, yet i don't think so that Ubisoft support will help us on this one honestly due to lack of inactivity on this OLD forum.
You bought FarCry3 Deluxe Edition on spring sale on UPlay, i bought FarCry3 Deluxe Edition on winter sale in 2017 on UPlay, we-and not ONLY we have the same annoying problem, and there is no way to retrive it back by ourselfs, for this we would have to earn help from Ubisoft Support wich is DEAD on this game forum, i will soon try to post our problem in FarCry5 forum or in populeted game/active forum like Ghost Recon Wildlands, they will not check this HERE, so we gonna have to break rules and post our problems on different forums to get their attention about this.
Alternatively, you can crack/download it from internet, there are even working 100% legit turtorials about it on YouTube, but this is like last effort.