Hello,
I solved it after endless tests.
I had also called Adobe support, which made me lose only half a day without giving real indications, they only made many unnecessary tests, on apple account computer etc. no tests on Dimension and in the end I was also told that my Mac computers had the graphics card has some problems and needs to be updated.
I said that I also had friends with the same file tested, and everyone sees the 3d .fbx file in Dimension faceted.
I also said that for everyone who imported the 3D file with other more complex software, my 3D file was perfect.
Only Dimension showed it badly.
In the end after many tests done by me, I discovered that Dimension has problems importing some forms on .fbx format
I solved it by importing the same 3d with another software and exported it in another format.
Thank you