Is there an eReader application that runs under Windows, Android and Linux flavors? Someone just gave me a tablet but I also use MS Windows and Linux Mint. I could wade into searching online but I presume most work under only one OS.
FBReader and Cool Reader support all the operating systems that you have listed, but I don't personally use them, so I can't give you more insights. You can try them and see if they fit your needs.
The e-reader that comes with Calibre works on Linux and Windows.
There is companion application that work with the database from a tablet, but doesn't have the reader, it launches any other one found on the device.
This probably is not the answer you are looking for, but I highly recommend using Google Play Books for cross-platform reading. It renders things so well that it's my default choice for everything.
I'm not criticizing FBReader or other readers which are cross platform, but many of these apps just don't render ebooks well. I have basically given up trying to format for them because they override so many publisher defaults. About the only epub reading app that doesn't override defaults is Adobe Digital Editions, but there's no Linux edition version -- though I suppose you could use WINE or something like that. (Curiously Adobe DE has a mobile android version which works pretty well though has minimal features).
There's a Readium plugin in Chrome that lets you view epub files offline in any Chrome browser.
Calibre is cross-platform too, though the reader is not particularly reader-friendly; it was built mainly to let you see the result of converted ebooks.
An app that overrides publisher defaults is the best recommendation for an ereader. The user knows what size fonts they want not the publisher– mmmmmmJun 15, 2016 at 0:47
Oh, I have had that conversation many a time with all kinds of people in publishing and ebooks. The user controls for these let-the-user-decide reading apps are primitive at best. (Such as: image size, borders around images, space between list items and blockquotes). A good publisher uses ems instead of absolute pixel size so that the user can decide the right font size. When I test ebooks, I test all possible font sizes. That's not the issue I'm talking about.... Jun 15, 2016 at 3:27
FBReader, Moon Reader, Gitden all have problems with rendering images consistently. I don't want to criticize because these have smaller development teams and a smaller user base. But Google Play Books let the user change font size and spacing while still supporting most css... Jun 15, 2016 at 3:37
For windows/linux platforms you can use calibre
For Android, the best app I've found so far is eReader Prestigio
Chrome users can use Readium extension