[1830] adds 3rd 6-train option

[philcampeau]

Fixes #10046

Before clicking "Create"

• Branch is derived from the latest master
• Add the pins label if this change will break existing games
• Code passes linter with docker compose exec rack rubocop -a
• Tests pass cleanly with docker compose exec rack rake

Implementation Notes

• Explanation of Change
  The Avalon Hill rules did include an option to add a 3rd 6-train. This is also referenced in the Mayfair rules:

[philcampeau]

No pins needed

[philcampeau]

I got a pic of the optional rules section. You're able to use all three changes listed, or any of the three. I don't think there'll be much demand for the others, but I can implement them too if desired.