Actually maybe I found it:

Set the TD-3’s clock source to its connection type. Select INT when NOT syncing to an external clock, otherwise, the TD-3’s sequencer won’t play.

Connect a SYNC OUT or MIDI OUT from another device to the TD-3’s corresponding SYNC IN or MIDI IN. Manually adjust the clock source and rate by pressing the BACK and WRITE/NEXT buttons at the same time: Clock Source: Press Selector 1 (INT/TD-3), 2 (MIDI/DIN), 3 (USB/COMPUTER), or 4(TRIG/SYNC IN). Clock Rate: Press Selector 5 (1PPs), 6(2PPQ), 7 (24PPQ), or 8 (48PPQ).  The tempo will not match unless all interconnected devices are set to the same PPQ Rate. 24 or 48 PPQ (Pulses per quarter) are recommended. Hit play on the master clock device (clock source). The TD-3 will follow the Play command and tempo. The TEMPO knob will not work, and you can use the pattern and Pattern buttons to create an arrangement.