You miss a bit of context if you start the AP story without playing Reunion or TC, but the main story isn't a huge part of the game anyways. If you're interested in it and don't want to miss out on continuity, I'd play Reunion with the XTended community mod first and then TC and AP. Fair warning though, I found Reunion to be a lot less casual-friendly than TC and AP. The random side-mission system was tied into station bbs and tended to reward you less, making it more tedious to make money early on unless you're good at capping ships.
If you're more interested in building an empire and such, I'd just start in AP since you can't transfer saves between any of the games.
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