Hi Matt
CS3
I would be VERY reluctant to purchase a second hand CS3 and the protective folie on the screen is not a testament it is brand new.
Yes, sometimes, the CS3 (60226) in my case is slow at recognizing MFX locos and especially at editing them. With the latest update 2.5.2(6) sometime I have to start editing on my laptop.
Since the update v2.5.1(0) this Märklin idea to change the "firmware" is OK for Märklin decoders but a disaster for non-Märklin ones (I have an ESU Loksound 5 Micro that, since the "operation" of "Updating the firmware has all its physical ourputs except the motor out of order. So beware with these 2 updates when using non-Märklin decoders.
For Märklin ones, it works rather well.

Remember that ESU M4 is NOT MFX: it is only for using the protocol but when it comes to editing it is a shipwreck.

Regarding the BR06, I have one and it is a splendid loco BUT:
- never use regular smoke fluid but only the Märklin LIGHT smoke fluid 02421. Because I used a regular smoke fluid (RS24 or Märklin 02420) the fan inside the smoke module seized which caused the heater to burn. Return to Göppingen where it stayed 3 month but was returned repaired at not cost (Märklin paid both shipments).
- in many cases, the failure cause the loco to draw 2 amps which may trigger a short circuit detection.
- the loco runs very well on R1 BUT (I didn't pay attention) I had a my layout an R1 curve section terminated by a triple switch. Big ERROR, one must insert a small straight section because otherwise it derails (but now not anymore with a 24077 section between the R1 curve and the R2 counter-curve.
- I had another issue with the drawbar between the loco and the tender. It needs to be set in extended position but in my case it was going to the narrow position, thus causing derailments in reverse mode. The solution was to add a bit of temporary glue NOCH 61121 (Hin und Weg Kleber= temporary glue)

Regarding MFX registration I discovered that disconnecting the Ethernet cable (I use Rocrail on a Win 11 laptop) make this much easier.

CS3 Failure
If your CS3 is totally dead though no fault of yours (no falling on the ground) you may return it to your seller (if purchased very recently).
I have experienced Märklin repairs (CS2 in Göppingen) on a permanent failure and the result was:
- a 3 month delay (France)
- a medium repair cost (under 200€)
- a brand new mother board which transformed my early 60214 into a 60215 of the latest make.
- we don't have the problem in the EU but outside, you must pay attention to the custom declaration so that you don't pay neither way.
- avoid sending back to Göppingen non-systematic failure items. It will last 3 month and be returned to you as it was, most of the time

Regarding the non-MFX locos addresses, the easiest is to:
- pose the loco on the track ALONE
- set the loco zone into edition mode
- select add a new loco and select either MM2 or DCC and enter ANY address
- the next screen will include a key to FIND the loco address. Push it and you are done (below circled in pink)


Here is my Märklin fun in operation (A CS2 at the time but now with a CS3 since more than 3 years)


Cheers
Jean