Part 1 (Supergirl):
a) A pre-wedding reception at CC Jitters, with Kara, Oliver & Felicity, the West clan, Barry, Cisco, Jax, Mick, and probably others.
b) The wedding itself, going beautifully until a bunch of Earth-X soldiers storm the place for some reason.
c) Boy, did those bad guys pick the wrong wedding to crash...
Part 2 (Arrow):
c) Oliver, Felicity, Barry, Iris, Sara, and Kara are still in their wedding clothes here, so I'm guessing it comes before the other photos. They're in the STAR Labs Pipeline entrance, where they appear to meet the Ray and Citizen Cold.
a) The heroes with some sort of capture collars around their necks, in a bunker somewhere. I'm guessing shock collars rather than power-dampers, since Sara and Alex have them too. We meet the evil counterparts, Reverse Flash, Overgirl, and Dark Arrow. It looks like the Flash gets loose and Green Arrow then shows up.
b) In what appears to be the STAR Labs Speed Lab (or its Earth-X counterpart), a confrontation including Dark Arrow, Wild Dog, and Heat Wave.
Part 3 (The Flash):
a) In the STAR Labs Cortex, Dark Arrow, Reverse Flash/Wells, and some Nazi grunts have Felicity and Iris captive. Overgirl appears to be there too. From the weird lighting, I'm guessing this is the Earth-X STAR Labs.
b) Green Arrow, White Canary, Alex, and Citizen Cold in a firefight in a warehouse or storeroom.
c) Oliver, Barry, Sara, Alex, Jax, and Stein in what looks like the Earth-X resistance HQ with Ray Terrill (the Ray), Snart, and stubbly alternate Winn. Terrill is wearing a Nazi prison uniform with a pink triangle, the symbol used by the Nazis for gay people. It looks like he's giving the heroes exposition about something. Later, they're gathered around a tabletop miniature listening to Oliver strategize. (This is still under "c," but it must be a new scene, since Terrill has suited up.)
Part 4 (Legends)
a) In a location I can't place, Kara looks on as Felicity shares a tender moment with... Dark Arrow? Or maybe Oliver in disguise as Dark Arrow? EDIT: Kara seems like she may be in hospital scrubs and leaning on Felicity for support. If Supergirl was weakened/captured somehow, that might explain why she's absent from so many of the photos from parts 2-3.
b) Big gathering on the Waverider bridge -- all of Teams Flash, Arrow, and Legends except Diggle and Firestorm, plus Supergirl, Alex, the Ray, and Citizen Cold. I'm guessing they're listening to Gideon or watching a hologram she's projecting, and they don't seem happy about it. Then the group spreads out and there's more business on the bridge, including maybe Cisco and Harry trying to pilot the ship somewhere -- is Cisco trying to reprogram the Waverider to breach back to Earth-1?
c) The Legends, Team Arrow, the Flash, Alex, and Citizen Cold standing their ground under a bridge, with a wrecked bus behind them. Probably the start of the big climax. I'm thinking they're back on Earth-1, defending their home against the invaders. No sign of Supergirl or Firestorm, but maybe they're in the air. No sign of the rest of Team Flash either.