The 8EX and 3DW are package designations, however when looking at devices from Microchip Technology you will notice that both the 8EX and 3DW are listed as 16-QFN (4x4), so what is the difference? The difference is in height, the 8EX is listed as 0.9mm in height while the 3DW is listed as 1mm in height.
These and other package designations can be looked up on the manufacturer website.
Package Outline Drawings | Microchip Technology
Applicable Part Numbers
|Manufacturer Part Numbers