The Duet 3 Expansion 3HC board connects to the Duet 3 CAN-FD bus and provides 3 high current stepper driver channels, along with heaters, fans and GPIO. Multiple expansion boards can be daisy chained on the CAN-FD bus.
Product page is here: https://www.duet3d.com/Duet3Expansion3HC
Documentation is here: https://docs.duet3d.com/Duet3D_hardware/Duet_3_family/Duet_3_Mainboard_3HC_Hardware_Overview
As always please discuss this on the Duet3d forum: https://forum.duet3d.com/