I remember some local councils over here used to use Amiga 2500s for CAD in the early 90's. Mostly road and curb designs etc. So there must be some good software somewhere. Dunno if it will work on 3.9 though.
I got DynaCADD 2.04 (4 Disks) It seems okay but complex and the instruction manual is useless. I could try installing it on my 3.9 A1200 and see if it works.