We use OneDrive to share documents across 3PCs. On one PC we can open everything with no issues. On the other 2, when we are trying to open the same Excel documents (they're not open on the first PC) we are getting the error message that Excel cannot open the file because the file format or file extension is not valid. Can anyone help

Was the document created on the PC that CAN open everything?

The basic assumption is that all 3 PC's have the exact same version of Excel - correct? Someone more fluent may know this, or it may be obvious, that is, can you have the same document open on all 3 PC's at the same time, or is that not the issue? Rather, is it simply you cannot open a single document, on a single PC, at one time?

This may not be an Excel related problem, some browsers change the extension of the file, try opening that file using another browser. Are you opening that file in browser, using Excel Online, from that mentioned OneDrive folder? Or you are downloading the file to open it with the installed version of excel ? Anyway, changing the browser may be the solution for both cases.
Cheers,
Catalin