If any kind of u3d to png conversion is even doable, it would be making a picture preview of some graphics content from ancient U3D files as a simple PNG graphics files. The result of such export will be just a normal picture of the 3D model saved in the .u3d file. Many tools actually support U3D format and can visualize it, even some versions of Photoshop did.
Updated: July 18, 2023