Before revision: A three-operand architecture (2-in, 1-out) will allow A := B + C to be computed in one instruction A two-operand architecture (1-in, 1-in-and-out) will allow A := A + B to be computed in one instruction, so two instructions will need to be executed to simulate a single three-operand instruction.
After revision: A three-operand architecture (2-in, 1-out) will allow A := B + C to be computed in one instruction A two-operand architecture (1-in, 1-in-and-out) will allow A := A + B to be computed in one instruction, but two instructions will need to be executed to simulate a single three-operand instruction.