So hillsides, channels, creeks, rivers, and so forth will all be experiencing record run-off levels. This is why the forecast calls for significant hillside failures: it will be more water on a hillside than many of those hillsides can bear. It will flood beyond the normal flooding limits. It might... be historic. Maybe. Huge if. But super-dangerous if it happens.
3 inches in the Olympic Peninsula would be less of a big deal because that's routine. All the hillsides that can fail with that already have. So you're right that 3 inches is not a big deal in most places. But on landscape where that doesn't ever receive that much, it suddenly is a big deal.
The fear I have with these recent atmospheric rivers is that we have a lot of snow in the Sierras now. If we get an atmospheric river which is too warm, bad things could happen[1]. Fortunately as we get deeper into Winter it's probably too late for that disaster to be possible.