You should start by reading about triple expansion engines. Study the work of Herreshoff; most of his work is available at the Hart nautical museum at MIT. You can get any of his planes, they are now on microfilm.
One thing you want to be careful of is the power stroke, a triple is set up High Low, Intermediate. The rod journals are not always 120 degrees. The high comes to TDC then the low and then the Intermediate. The steam goes from the H to I to L. Since the valve is not open for the steam exhausting from the H you need a large manifold to store the expanded steam till the valve opens. This is three to four times the cylinder volume of the cylinder it is being exhaust from.
I know where you can get a set of casting for a compound 3-1/5 X 7 X 5 all piston valve.
This is a three-cylinder compound I designed, not a triple.
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Rolly