I can also confirm that the piece system (ie, 32kg x 2, plus 32 x 1 for *G) applies for the whole RTW itinerary.

If an agent gives you grief, another point of reference is the fare rules for the RWSTAR tariff, which explicitly states that "PIECE SYSTEM SHALL APPLY FOR THE WHOLE JOURNEY" somewhere in the fare rules. You can ask the agent to refer to the fare rule if s/he refuses to allow you the 3 pieces even after checking your ticket (which already says 2PC).