Hi Trev
So your CS3 is stuck in Stop mode while all is disconnected except the power supply
You may have jammed the STOP button which presses on a microswitch directly soldered on the mother board.

What can you do?
- make sure the STOP taste is not jammed down (does it have its regular play between the UP and the down position. On a CS2 I had a STOP taste jammed and I had to unjam it.
- connect to "Tante M" using internet to fetch for the last s/w version. In some cases it may clean files and solve the issue (get a new GFP (GleisFormatProcessor)
- as a last resort, if the CS3 is not under warranty anymore (2 years after purchase) you may try to open it to check if a plastic part is broken. You need a long Torx wrench and go pasdt the warranty seal

I never opened my CS3 but intervened on my CS2 (almost the same casing and probably a similar shape for the mother board).

On the mother board there is another plastic part to interface with the surface-mounted device switch Here it is (part view of a 60215 motherborad rear side)


NOTE: Märklin will NOT sell any inside spare part so either you repare it (glue broken plastic part, replace the SMD switch if falty with proper SMD soldering station (hot regulated temperature air) or you send it to repair in Germany.
All the above is IF the STOP button does not work.
If the short detection is faulty reload the S/ and make sure the programmed power supply in you CS3 MATCHES the reality (It changes sometimes mostly after an update)

Cheers
Jean

Not quite true. I managed to break the clear plastic part in the above photo, on one of my cs2 units. I took a photo of the broken part and sent an email with the photo attached to marklin service, saying I found my stop bar didn't work and this is the reason. They wanted me to send the whole cs2 back for repair, but i protested that the fault was obvious and I didn't see a need to go to that expense. They eventually sent me the plastic piece gratis.

Double check if the type of power supply entered on the CS3 has not changed. If incorrect it may trigger an overload situation
Jean