Need a T-Piece to connect two DN50 pipes and have in run into the third one to the sewage...

Here you go: Simply print this (Recommendation is ASA, to be tight and strong) Insert the DN50 Rubber seals into the ring, add some clamps to avoid breaking the Piece while inserting the connection tubes.

I use it in a lifting-station-case, on the one side the washing machine is attached and on the other a floor drain.

By the way: You will not get this at the plumbing store.