Please make sure that you have updated your Galaxy Watch3 to the most latest software.
Kindly follow these steps:
1. Head into your Galaxy Watch3 Apps Tray and launch Samsung Health.
2. Scroll down the list of options and tap on Blood Oxygen.
3. Select Measure.
4. It is recommended to place your hand on a table and near your heart. Sit still while your watch measures your Blood Oxygen level.
5. Once measured you will be able to view your Blood Oxygen level and Beats per minute (bpm).
Hope that helps! You can also call the service center to get remote assistance on this too.