wmm to 3mm conversion is most likely related to export or conversion between two types of video content from 3D Movie Maker software. This is a case of exporting older format to newer one, since .3mm files replaced .vmm in later versions of 3D Movie maker.
The problem with this is that the software has been discontinued years ago, so it might not be easy to find proper version for such export. But there seems to still existing thriving community online for 3D Move Maker, so perhaps they can help you out.