Explanation: Capacitor 3 is the emitter bypass capacitor. In a common-emitter amplifier circuit like Figure 3C5, the emitter resistor (R4) provides DC bias stability. However, it would also introduce negative feedback for AC signals, undesirably reducing the amplifier's gain. Capacitor 3 provides a low-impedance path for these AC signals, shunting them around R4 directly to ground. This effectively "bypasses" the emitter resistor for AC signals, allowing the amplifier to achieve higher gain while maintaining stable DC bias. Capacitors 1 and 2 are coupling capacitors, blocking DC offsets while passing the AC signal into and out of the amplifier stage, respectively. Capacitor 4 is a decoupling capacitor, used to filter noise from the power supply line, which is a different function than directly bypassing a component within the signal path.