Arrow and the Flash take place in an alternate earth. You don't believe me? Look at the evidence: 1.Laurel Lance never dies in the original DC Universe. 2.Kid Flash and Iris are both white in the original DC Universe.Anyone who's watched Young Justice KNOWS that. 3.Superman and Batman are the founding members of the Justice League of which Black Canary,the Flash,and Green Arrow are members of. 4.Speedy is a GUY in the original D.C. Universe.

So looking at the evidence,it's clear that Supergirl is the ONLY show that takes place in the original DC Universe.