There are now four DConTV series; Arrow, Flash, Constantine and Gotham. We know that Flash and Arrow are connected, not least because Flash basically premiered on Arrow. But we don't know whether the other two are connected at all. Considering how Constantine is largely concerned with magical dangers rather than super-powered ones, maybe they wouldn't mesh well.
In Arrow, Detective Lance's call sign is Delta Charlie Fifty Two or DC52. When this was originally spotted, people thought it was just a little shout out to the most recent DC reboot, the nu52. There are several other references in Arrow to the number 52, including Deadshot staying in room 52 of a hotel Cyrus Vanch being suspected of 52 murders.
Now, if this was just one series we might dismiss it.
In Flash, we constantly see these signs in the pipeline (the part of the particle accelerator now used to house captured metahumans) that have the number 52 on them. We see them almost every time the team lock a villain up in the pipe and, like in Arrow, they're never directly referred to.
Well, they're two series that we know are set in the same universe so no problem here, just a recurring joke.
During episode 5 of Constantine "Danse Vaudou", we see one of the cars in the background featuring the number plate NU5 2C3 or NU52, which is, of course, referencing the DC reboot NU52, just like Arrow and Flash.
So we have three DC owned tv shows all sporting the visual same references. Thus far, there haven't been any references in Gotham but we can hope. Does this mean they're connected and we'll one day see John stare down Barry and hopefully blow smoke in Oliver's face? I don't know about you but I hope so!