Marvel is ruling the movies at the box office, and is now getting into TV shows with Netflix and its connected universe with its films. DC is kind of going in a different direction as it is doing well on TV, not so much in the movies, which are not related at all to its CW TV shows. Arrow was a hit television show for DC on the CW, and it eventually spawned The Flash, which was also a hit. Both shows were such hits that it spawned a team show called DC's Legends of Tomorrow, and now Supergirl, originally on CBS, is going to join this team.
What happened at the end of The Flash is that Barry Allen travelled back through time to save his mother's life. This resulted in a restructuring of the entire timeline, and it has been revealed that Season 3 will focus on Flashpoint. Flashpoint is a DC comics event which restructured the DC universe, and there is an animated version of it if you would like to see what it might be like for The Flash Season 3.
By the way, there is report on Comic Book saying that Flashpoint is not going to last all season long. No doubt time travel and altering reality is going to affect other elements of the CW DC Universe as DC's Legends of Tomorrow ended with the Time Masters not being able to police the timeline.
One thing that you will learn about the DC Universe, it has a tendency to alter itself. It's a good way of getting rid of the bad writing choices of the past and keeping what made the comics great in the first place. The big event in 1985 known as Crisis on Infinite Earths, for example, created a DC Universe that had no more alternate timelines.
By the way, alternate timelines were a huge factor when there was a crossover on CBS between The Flash and Supergirl. At the time, they didn't exist in the same universe, kind of like The Flash and his Jay Garrick counterpart. It's pretty clear that by the middle of 2017, when these shows have finished their respective seasons, Supergirl will be a part of it.
Not just Supergirl, but Superman as well. Yes, Tyler Hoechlin will be appearing as the Man of Steel, a part that really should have been a part of the first season. Another thing that is planned to appear is LuthorCorp, and there is going to be a Lena Luthor appearing on the show. So is Lex on the way? Probably.
The weirdest thing about the show is how Superman is becoming a side character in this DC CW Universe. I can't help but compare it to Spider-Man's role in the Marvel Cinematic Universe. As in Spider-man is like Marvel's biggest character, but he shows up in Captain America's film as a side character.
Let's see how Superman fares, and it is very great to see CW create four TV shows that take place in the same universe. Hopefully they have a big plan for this. All of these shows will be back in the fall.