Sorry for the late reply. Will be happy to help you. If it’s still needed, can make a short video for better understanding:)

Man.... I feel so sorry... I just realized that I didn't get back to you. You just need to connect it with MIDI cables... MIDI Cable Connection: Connect a MIDI cable from the MIDI OUT of the TR-8S to the MIDI IN of the TD-3-MO. MIDI Channel Settings: Ensure both devices are set to the same MIDI channel or make the TR-8S the master and the TD-3-MO the slave. On the TR-8S, press [UTILITY] > [MIDI] > [SYNC OUT] to set it to transmit MIDI clock. On the TD-3-MO, set the MIDI channel by holding the FUNCTION button and turning the MODE knob to select the desired MIDI channel.(Not sure about this, never used "MO" version before, but that what google advised me) Clock Settings: On the TR-8S, set it to send a MIDI clock signal to synchronize the TD-3-MO's tempo. On the TD-3-MO, ensure it is set to receive an external clock. Audio Cable Connection: Connect an audio cable from the audio output of the TD-3-MO to an audio input on the TR-8S (if available) or to your mixer/audio interface. Sorry again for such a late reply...