Final answer:
The temperature T3T3T_3 of the gas in state 3 is equal to T1T1T_1.
Step-by-step explanation:
In an ideal gas, the temperature remains constant during an isochoric process, where the volume is held constant. Therefore, since the gas is in state 1 at temperature T1T1T_1, and there is no change in volume, the temperature at state 3 (T3T3T_3) will also be T1T1T_1. This is based on the ideal gas law, which states that for an ideal gas, the temperature remains constant during isochoric processes.