[REL] Schwimmen (2018)

[Harry789]

Hmm, there used to be ARD (channel 1) and ZDF (channel 2), but which one they call "channel 3"

Your are right with ARD and ZDF. But Channel 3's are local TV stations.
Every federal state in germany has it's own channel.
For example: channel 3 in Bavaria is called "BR Fernsehen", channel 3 in Baden Württemberg is called "SWR Fernsehen", and so on.
This is from a time when there were only three television channels in Germany, ARD, ZDF and "Drittes".