"he did a simple mod of putting a pair of very light springs on the valve nuts to keep the valves pressed against port" --> No need to change the steam engine..
To running test for each cylinder: Remove the two connecting rod to other cylinder who is not in use (due less load caused by friction), the cylinder who is under test/adjusting timing must have connecting rod to run the steam engine. Also you are adjusting/running the triple steam engine as if it were "one cylinder steam engine".
After each cylinder has correct timing and running test for each cylinder is done, mount all connecting rod and the two manifolds on place and the triple steam engine is ready to run.
Here are a ton of good tips in this link.. Stuart Triple Expansion Engine
Here is my triple expansion steam engine running at airpressure.