I found a workaround for this issue, but am posting this here as both a bug report and guidance to others who run into the same problem.
I found that Dorico 3.0 would consistently freeze midway during an export to WAV. (Always at the 26% point for some reason.) A freshly-created Dorico project didn’t have the same problem and would export fine.
The project had two flows, both of which were attached to the same video file. I tried detaching video from one of them, at which point the export to WAV resumed working correctly.
So: it might be the case that D3.0 has trouble exporting to WAV when more than one flow has video attached.