In latest iTwin Capture versions we see movement to remove 3mx. This is mistake. Please keep support of 3mx. 3mx format provides direct publishing to web with best webGL 3D viewer (thanks to Acute3D) which works on multiple platforms PC, iOS, Android.

Also 3sm doesn't support local coordinates(meters/mm). Architects or designers doesn't model in real coordinates because of solids working area issues or UTM scale factors.

3mx is open and compact format and well supported. 3sm is not replacement and wastes HDD space as direct publishing requires to convert to different formats. Also iTwin web viewer is too bloated, not accesible from non-Bentley accounts and not supported well on different platforms.