I learned C on a 3B2, and for a while had access to a 3B20S at UIUC. You were right about the 3B20; UIUC had a pair of them that were originally called "shark" and "tornado", but were quickly nicknamed "snail" and "turtle" in honor of their performance. They were the size of refrigerators, and, IIRC, they were roughly equivalent to a VAX 750 (maybe a 780 at best).

To the best of my knowledge, the 3B20D, a dual-processor fault-tolerant system running a UNIX variant called DMERT, was only used in the #5 ESS telephone switch - and was eventually replaced by an emulator running on a Sun system.