Hi Mailme (a first name would be great)
A warm welcome to this great forum where most of us provide an introduction ("Presentation"). This is not police stuff but rather to get better acquainted.
Here a Märklin fan from Ile de France (greater Paris) with HO, digital, Rocrail to control trains through a CS3.
A normal reaction of the CS3, when posing a loco on the rails, is to try connect using MFX (Top-most priority for Märklin) even if the loco is already connected using DCC.

Your choices:
- you may block the MFX protocol at the CS3 level (it would be a bad restriction on something very powerful). I authorise all 3 protocols
- you may block the MFX protocol at the loco level (depends on your decoder type). Märklin uses CV50 to block/authorise protocols. This helps barring the loco from any MFX registration



A CS3 works great with Märklin decoders where it is the MOST user-friendly (the right icon, all the used fonctions, etc).

Notes:
- recent versions of the CS3 (2.5.1 and 2.5.2) have created issues with non-Märklin decoders. So beware. Märklin says it changes the firmware, I fdon't know but one decoder I have ESU, Loksound5 micro has been "modified" by the CS3 and now I have no more headlights.
- I believe it is best to disable the unused protocols at the loco level (depending on which decoder brand you are using the disabling may change)
- remember that you cannot disable the protocol you are presently using to talk to the loco (so you may want to connect using DCC to disable the MFX protocol) and then delete to possibly created MFX registration.
- avoid using MFX on accessories like 60832, 60842, MFX signals because it is best to remain in charge of assigning all addresses

Here is my layout in operation with digital and Rocrail (at the timle with a CS2 but since 3 years with a CS3
https://www.youtube.com/watch?v=F6WnhXPcRqI

Cheers
Jean

Hi


No. you are absolutely right (and I have done it but forgotten). YES you must first disable (temporarily) the MFX protocol on the CS3. This is the only way to "talk to the loco" in DCC.
Then disable on the loco the MFX protocol, remove any possible MFX registration on the CS3
Finally you may authorise back the MFX protocol because on recent Märklin locos, it is so much more convenient.
Cheers
Jean

So rude of people to come in here asking for help and after we take the trouble to try and offer up a solution the response is crickets.