Based on the expanding renewable energy capacity and growing power system flexibility demands

load aggregators (LAs) have emerged as critical entities bridging demand-side resources and power markets

demonstrating increasing significance in multi-market coordinated operation. This study addresses insufficient demand-side flexibility and limited market synergies in traditional demand response by establishing a market framework that incorporates thermal power units

power sales companies

and LAs. To optimize LA's segmented electricity market bidding

bidirectional reserve market capacity configuration

and carbon allowance trading

a bi-level game model for their participation in the day-ahead electricity-reserve-carbon joint market is constructed. The model is solved via a mathematical program with equilibrium constraints (MPEC) transformation based on Karush-Kuhn-Tucker conditions and strong duality theory. Verification on a modified IEEE 9-bus system confirms that carbon market price signals drive LAs to optimize consumption—reducing peak loads and enhancing reserve market participation—while the synergy mechanism guides dynamic resource transfer between markets

inducing nodal price asymmetry. Increasing the proportion of reserve capacity exerts downward pressure on electricity prices

achieving "source carbon reduction" on the load side and providing new momentum for system-level decarbonization.