Unfortunately, ultrasound imaging device manufacturers often do not let users access 3D image data that they acquire. Users of most imaging modalities (such as CT, MRI, PET, etc.) would not accept this absurd situation, but for some reason ultrasound users got used to this and don’t raise their voice. The result is what you are experiencing now: you acquire 3D image data but you can view and process it only by using the manufacturer’s own software.
The best solution would be to stop purchasing 3D ultrasound equipment if it does not give you access to the data that you acquire. That would be the only thing to motivate ultrasound device manufacturers to change their current practice.
You can try to contact Canon to get access to their Image3dAPI codec and then you can use SlicerHeart extension to import 3D ultrasound data.
Another option is to do some manual work to figure out the image format by trial and error using RawImageGuess as shown here: