I've never heard of WaveRT (I'll look into it to satisfy my curiosity) but I can tell you about AC3.
AC3 stands for 'Audio Codec 3'. It is Dolby's internal name (like a code name) for Dolby Digital. It's kind of like how Windows Longhorn will be launched to the public as Windows Vista.
When the format was made available to the public, it was launched as 'Dolby Digital'. So you could say that AC3 is the official in-house name for the codec and DD is the marketing term.