What would be the best way to keep my collection (in epub/mobi/pdf) formats uploaded online so I can then read them from my 3 devices - Android phone, old Kindle tablet (which has google play store added) and pc.

Right now I am using Google Play books, but wondering if there's something better. I'd like to keep using Play Books as the place to upload, maybe use Google Drive as its reliable.

Also I am not able to upload my audiobooks into Google. What do people use for those?

Maybe you can try Moon reader apk

Maybe you can try Moon reader apk

I had to comment here for some reason.
Anyway, I use Pocketbook ereader from Google Play with Google Drive. It`s by far the best ereader and I have tried then ALL. I download my ebooks ( epub/mobi) save them to Drive. Pocketbook has its own cloud as well and it`s syncs across all devices. I have over 2000 books on there lol

Maybe you can try Moon reader apk

I had to comment here for some reason.
Anyway, I use Pocketbook ereader from Google Play with Google Drive. It`s by far the best ereader and I have tried then ALL. I download my ebooks ( epub/mobi) save them to Drive. Pocketbook has its own cloud as well and it`s syncs across all devices. I have over 2000 books on there lol

Oh.. and Pocketbook is free