The newest Galaxy Watch 3 update improves the performance of the blood saturation feature. The Galaxy Watch 3 is already highly regarded to be a very good watch.
Before, the update had rolled out in Samsung's home country, South Korea, but has now started to make its way to other countries too. If you haven't got the update yet, you may want to manually check. If you still don't have it after that, I guess there's a bit more waiting you have to do.
As well as the important for the O2 stat readings, there are minor fixes somewhere.
This update has the firmware version of R840XXU1BTK1. It also states in the changelog that this update improves the blood oxygen measurements. The heart race voice guidance is also mentioned within the changelog. This feature has been added when running and biking exercises rather than using a paired earbud.