DUST COLLECTION ADAPTER 90MM PVC TO 75MM SPIRAL DUCT.

This is one of a series of dust collection ports that can be found at this link:
DUST COLLECTION FITTINGS.

This is a simple adapter which allows a push fit, counter intuitively over the OUTSIDE of a 90mm PVC connection fitting, with a screw to suit three inch flexible ducting.  

The  mix of dimensions is not intentional, that is just how these products are sold in Australia.   The flexi duct is sized happily to fit inside the PVC joint, and it is beneficial to screw it a little beyond the length of the adapter to assist with the seal.

Once again, a few turns of plumber's PFTE (teflon) tape will assist in keeping an airtight joint that can be undone if need be in the future.