To diagnose what might be actual source to your problem, it might help to readjust your thinking about how the Google Photos service functions in the background. It's a misconception that the syncing is taking place directly between your phone, your tablet, and your PC. With Google Photos, the primary, central storage of your photo library is within your online Google account. It's your online Google Photos service that your phone, tablet, and PC connect to when syncing with each other.
So when you take a picture using the Camera app on your phone, that photo file gets stored in your phone's storage media, then the Google Photos backup/sync service eventually uploads that photo file into your online Google account. At that point, your tablet and PC use their Google Photos service to view/access that photo file stored within your online Google account. All three of your devices rely upon your online Google account for your photos -- they're not interacting with each other, they're all just tied to that same Google Photos storage online.
It sounds like you need to figure out if the problem is with your phone, or with your tablet and PC. Try taking a test photo on your phone, wait a few minutes for the automatic uploading process to take place, then using a web browser to access your online Google Photos account go to:
https://photos.google.com
(Don't use the Google Photos app on your phone, that's where the photo file itself is located.) If you see your test photo using a web browser to remotely access your account, that's an indicator your phone is OK, the problem is likely with some glitch using your tablet and PC. Conversely, take a test photo using your tablet and see if that gets automatically uploaded into your online account without a problem. One or the other should point out the syncing problem but if both are working out OK, that does indicate a more involved problem.
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