They're good, but I messed up my watching order for them. These three OVAs (2014-15) show what was happening with the different witch groups between Season 2 (2010) and The Movie (2012). What's more, they'd have worked very well there. They're expanding the world and its cast by showing us what's happening elsewhere. (Miyafuji only appears in cameos after the end credits, if at all.)
1. "ST TROND'S THUNDER" (Sept 2014, 29 minutes)
The German witches have been stationed in Belgica. That's Minna, Barkhorn and Hartmann, plus an additional Karlsland witch with big boobs called Heidemarie W. Schnaufer. Hartmann's twin sister Ursula then turns up with presents for everyone, which start with comedy dresses and conclude with a 50 mm monster gun that could probably launch you into space. It's so big that you need a special Jet Striker unit just to be able to carry it, so Ursula has one of those too. Ursula is the Strike Witches' equivalent of Ian Fleming's Q.
Hartmann's hostility to all this new kit is funny. She takes it too far, though, both verbally and in her attitude during a flight test.
It's a good episode, but I'd point out to the voice cast that if your character's supposedly taking a deep breath in, you shouldn't be saying "SU". Yes, that's a Japanese onomatopoeia that could be used in that context in manga (strictly meaning a quick or silent action), but the voice actress is obviously just saying the word and hence expelling air.
2. "GODDESS OF THE AEGEAN SEA" (March 2015, 26 minutes)
This one starts in Italy, with Shirley and Lucchini, but it soon moves to the Greek island of Delos. (Delos is real, incidentally, and is as important as it's presented here, culturally and historically.)
The weirdest thing about this one is that it's got a high-up military man who's not a dick. He's intelligent, flexible and heroic. He jumps into the action at a key moment to help the witches. He's the only man we've ever seen in this show who's not an arsewipe... AND HE'S ROMMEL. His first name's Ernst, not Erwin, yes, but he's Field Marshal Rommel, commander of the African forces. (The art isn't a particularly good likeness, but it could be him.)
Admittedly, everyone in this show is named after a real war hero. Hartmann and Barkhorn are named after German WWII aces Erich Hartmann and Gerhard Barkhorn, the first and second highest scoring aces in history. That's less obviously true of the Japanese characters, though, e.g. Miyafuji's apparently named after WWII ace Kaneyoshi Muto. Dunno how that works.
Everyone else, though, is gender-swapped. Rommel's just Rommel. It's the man himself. Weird. (In fairness, though, Rommel's the German general everyone likes, including his enemies. The British called him the "good German". He fought a clean war and was forced to commit suicide by the Nazis after being found guilty of involvement in the 20 July plot to assassinate Hitler.)
3. "ARNHEM BRIDGE" (May 2015, 31 minutes)
This is a Perrine episode, with Lynne in tow. Two children are found near her estate in Gallia, one (Rose) being on the brink of pneumonia and the other (Julius) having a bit of an attitude. Julius's aims and goals are honourable and he'll repay good deeds, but but he's also a brat who'll be a pain in the arse if he takes a dislike to you. In defiance of logic, he blames witches for his father's death. At one point, he kicks scaffolding and brings down a load of bricks upon his head, falling from about twenty feet up.
In other words, he's an idiot. He and Perrine clash, unsurprisingly. (They're similar, actually. They're both good people, underneath, but not blessed with the best of personalities.)
I liked this little series. All three episodes are worth watching. They do their job nicely of leading into the movie, with even a very small amount of overlap at the end of ep.3. The fanservice level is acceptable. (Some bath time and nipples in ep.2, but the panty shots are inoffensive.) If only the whole franchise were this good!