The PAM8403 datasheet says the speaker can be 4 ohms or 8 ohms. Yours is 3 ohms which is close to a dead short. The amplifier protects itself from a shorted output by disconnecting its output which might happen with your 3 ohm speaker. Or the amplifier might overheat.
Connect the two wires (audio and ground) of the mono output from your sound system to one channel input on the PAM8403 and connect your subwoofer speaker to the output of that channel. Do not bridge anything, you cannot increase its small output.
Connect the two wires (audio and ground) of the mono output from your sound system to one channel input on the PAM8403 and connect your subwoofer speaker to the output of that channel. Do not bridge anything, you cannot increase its small output.