As you have a Pixel 3 which still has support for high quality/storage saver photo backup:
From: Google Photos Help page - Choose the backup quality of your photos & videos
Photos and videos backed up in Original quality on or before January 31, 2022 remain in Original quality at no charge. Photos and videos backed up after January 31, 2022 are saved in Storage saver at no charge. Photos and videos backed up in Original quality count toward your Google Account storage.
Change your backup quality
Important: If you're out of storage, you can't change your backup quality.
- On your Android phone or tablet, open the Google Photos app Photos.
- At the top right, tap your Profile picture or Initial.
- Tap Photos settings Settings and then Backup and then Backup quality.
- Select a backup quality.
I just checked this on a Pixel 2 with Android 10, Google Photos app version 188.8.131.529567106 and was able to reach the setting.
Okay but how can I recover space in my Google account?
Prior to attempting you should archive outside of Google cloud your photos & videos before attempting to recover space in your Google account. You can use Google Takeout to archive just your photos and videos so that should any issues arise there is a copy not on Google's servers.
Alternative you can use Google Takeout to save your photos and videos outside of the Google cloud and delete manually to recover space.
Always check that the backup contains the data you want and the backup data is viewable/playable.
Google Photos space recovery from:
Choose the backup quality of your photos & videos - Computer tab toward bottom of page
Reduce the size of your photos & videos
If you have photos in Original quality and want to save some storage space, change your already backed-up photos and videos to Storage saver (previously named High quality). This setting won't appear if you have unlimited storage for Google Photos.
You can only recover storage once a day. Storage recovery won’t change the quality of your future uploads.
In some circumstances, certain photos and videos may not be able to be compressed and will be stored at their original file size.
On your computer, go to photos.google.com.
At the top, click Settings and then Manage storage.
Under “Recover storage,” click Convert existing photos & videos to Storage saver.