Gregg,
I understand it can be a little confusing. The DFF3D is classified as a "Multi-beam Sonar" where as the DFF3 and Internal sounder are classified as "Sounders". You can only display one "Sounder" (Internal, DFF1, DFF3, DFF1-UHD or BBDS1) module at any one time on a single MFD. However since the DFF3D is not classified as a "Sounder" you can display it and a another "Sounder" module.
I hope this helps.
Snips