**Unfortunately not**

For a long time Deutsche Bahn offered the so-called "tarifliche Gleichstellung", which grouped the stations of a city into a larger virtual station (e.g. Berlin Central Station, Berlin Südkreuz etc. were grouped into the BERLIN virtual station) - a ticket from/to such a grouped station was valid from/to all stations in the group. However, they stopped offering it for long-distance tickets in December 2023. Since then you're required to adhere to the routing printed on the ticket. So:

> 1. EC379 starts at the central station. Can we use the ticket as is just from there?

No, you have to enter EC379 at Berlin Südkreuz. The ticket is not valid between Berlin Central Station and Berlin Südkreuz.

> 2. If no, can we use the ticket as is for a non "Zugbindung" train (S-Bahn, Regionalbahn) to get from Central to Suedkreuz (instead of starting at Prenzlauer Allee)

Although the routing for local transport is less strict than the routing for long-distance trains, you're still limited to sensible routes without unnecessary detours. Unfortunately there is no sensible route from Prenzlauer Allee to Südkreuz via Central Station - so you cannot use the S-Bahn from there either.

> 3. Do we have to buy a separate ticket from central to Suedkreuz?

Basically yes, but you could save some money by buying short trip tickets to either Friedrichstraße (via the overground S-Bahn lines) or Brandenburger Tor (via U5). Both stations are on a sensible route from Prenzlauer Allee to Südkreuz, and as such you can use your train ticket from there.

Aside from that: the trip between Central Station and Südkreuz takes about five minutes, making it unlikely that your ticket is getting checked that soon. Even if you're getting checked right after Central Station, there is a good chance that the conductor lets you get off the hook with a warning if you're being reasonable about it.