Most steam sound decoders have an adjustment for the 'chuffs' per driver revolution. Normal steam engines woul have 4 'chuffs' - a 3 cylinder engne would have 6. Some of the high end decoders also have an input to allow a driver attached cam to control the chuffs. Where there might be an issue is if that 3rd cylinder is larger and 'sounds' different. Some decoders have an 'articulated' setting as well. If the sounld is galloping, I would suspect that the valve timing on the steam engine were not set correctly!
Jim
Modeling BNSF and Milwaukee Road in SW Wisconsin
They had a distinct 'galloping' sounds that made them sound as if they were moving quickly when they were just switching the yard. Any of the programmable sound modules have available sounds for this?
Thanks
NC