Right now, you can log onto Origin (or create an account), click on one button, and own Battlefield 3. It's that easy.
As a part of Origin's On The House program, which offers a new free game every month, Battlefield 3 is free for the next week or so. There's usually a bit of an overlap with its free games, and there's no rush, but I'd jump on it nonetheless so you don't forget. I'm actually a little surprised to see them offer something so recent — up until now it's been the likes of Dead Space, Battlefield 1942, or Plants vs Zombies (which is still free right now, by the way).
In fact, Battlefield 3 is quite similar to Battlefield 4. There isn't that much of a graphical difference, and even though it doesn't have the dynamic levels, gameplay is much the same. If you're on a budget, and looking at a free Battlefield 3 vs a full-price Battlefield 4, it's an easy decision. It's like getting last year's version of FIFA, if the roster changes didn't factor into it.
You can head here to get Origin, if you don't have it already. If you're like me, and you're in the office, you can click the button that sticks the game on your account and then click 'Cancel' on the auto-download.