This document, based on GRPE/2026/4, presents Amendment 3 to UN GTR 21 on determining system power of hybrid electric vehicles and pure electric vehicles with multiple propulsion energy converters. The GTR establishes two equivalent test procedures (TP1 and TP2) to measure system power comparable to traditional engine-based ratings. TP1 measures electrical power from the rechargeable electric energy storage system and estimated internal combustion engine power, applying electrical conversion efficiency factors. TP2 measures torque and speed at powered axle shafts or wheel hubs, applying mechanical conversion efficiency factors. The regulation requires manufacturers to provide hybrid power flow descriptions, energy conversion factors, and reference points for power measurement. Testing occurs on chassis dynamometers, hub dynamometers, or system bench facilities at fixed dynamometer speeds corresponding to maximum power conditions, with five repetitions conducted and results averaged. Applicability of each procedure depends on powertrain architecture and power flow characteristics. Amendment 3 introduces system bench testing as an alternative, strengthens measurement requirements, requires manufacturer-provided K factors rather than defaults, and establishes reference point concepts to ensure consistent results across different electrified powertrain architectures.