Add three-member short alliance prototype

docs/RULE_DECISIONS.md

@@ -1,5 +1,12 @@
 # Rule decision index
 
+## Three-member short alliance option — 29 October 2026
+
+The authorized revised rulebook, physical p. 42 (printed p. 40), lists a
+three-member limit with a three-stronghold target separately from the
+four-stronghold and unlimited options. The staged Basic variant retains the
+existing provisional two-controller precedence. [Scope](OPTIONAL_ALLIANCE_VICTORY.md#three-member-three-stronghold-variant--29-october-2026).
+
 ## Sapho and simultaneous battle Truthtrances — 29 October 2026
 
 In the provisional online ordering, a clean unrevealed battle retains its