Reviews

The OVA for Attack on Titan has three unrelated episodes that all happen while Eren and friends are still training. The episodes are all quite different in theme. The first one adds to the information you have about the world, the second one is attack on titan's attempt at comedy and the third one is mainly just a neat story. The animation, voice acting and music are the same as in the main anime.

At the end of the opening the episode number is given which could indicate in what order the stories happen:
Episode 1 is listed as episode 3.5
Episode 2 is listed as episode 3.25
Episode 3 is listed as episode 3.75
Do not take this as a watch order though. While episode 1 and 3 can be watched between episode 3 and 4 of the main anime, episode 2 of the OVA contains spoilers for something you'll only know when you reach episode 9 of the main anime. Watching the OVA after finishing season 1 is a perfectly fine experience.

Episode 1 is a story about a discovery the scout legion makes and gives more information about how much is known about titans. It's a nice episode to watch if you're interested in the world.
Episode 2 is a episode in which everything revolves about humor in a shokugeki no souma like cooking battle. I didn't find it funny and the characters didn't feel suited to the comedy but it can be worth watching if you like comedy.
Episode 3 is a story about a training mission which is mainly about character dynamics and overall neat to watch.