Seems like everyone's hit the major points, but guess I'll put in my two cents as well.
Download the Extended Cut. Its free, and while it doesn't make the endings the best, it at least makes them tolerable and gives closure. If you only do one thing, make sure its this!
If you want, try to get either Javik or the Leviathan DLC, or both. Javik is a Vanguard type character with some pretty interesting stuff to say, and Leviathan gives some more insight into Reaper Origins as well as making the ending a bit easier to swallow.
You will meet James Vega. Give him a chance. Not the best character, but definitely not the worst one. Certainly miles better than Jacob.
Try and think about which combination of party members would give the best banter during missions. The dialogue between the members has gone up drastically compared to the other two games, especially on the Normandy. Javik and Liara have a great one towards the end of the game.
Multiplayer isn't too bad. You might want to at least give it a try.
This game is super emotional. So prepare yourself.