Okay, basically Sins of a Solar Empire has had three major expansions. Entrenchment, Diplomacy, and Rebellion.

Trinity is a combo pack of Original, Entrenchment, and Diplomacy. It doesn't have Rebellion, so there's no reason to get it unless you have friends you want to play with who won't update to Rebellion.

Rebellion has everything Trinity has, plus is added far more content than the previous expansions. So for people wanting to get into Sins, its what you should want.

Forbidden Worlds and Stellar Phenomenon are standard DLCs for Rebellion (not trinity). As mentioned they make the galaxy a bit more diverse and add a little bit more 4X elements. You do not have to have them to play Rebellion mods or multiplayer however, so there's no reason for you to get them right away. You can try Rebellion first and if you want more variety/4X elements in the game, get the DLCs.