Noting down some saved information
From gearslutz :
To sync the Nord Lead 3 arp to midi:
“Connect your midi cables, out of sequencer (or clock source) to input of the NL3
Press the MasterClock button (configuration buttons) and select EXT as a source clock (instead of INTERNAL)
Press RUN to start the arpeggiator – the green light will go on (the rate will be controlled by the external clock source ie. your sequencer).
Then press the SHIFT AND THE ARP RATE KNOB left or right to set the timing ie. 4, 4T, 8, 8T etc.
The red light below should be lighted and your arpeggiator will play back in perfect timing with your sequencer.
… there seems to be no mention of this in the manual.”