Welcome to Opera, I love it.
So, are you trying to import bookmarks and passwords FROM Chrome and Firefox TO Opera?
When you first install Opera I think it gives you a chance to then, and that may even work better than the settings > import approach. Not sure, however.
I wonder if Chrome and FireFox also utilize the same file structure as Opera. Inter-compatibility may not be an option, equally the fault of each product, but hopefully for simplicity it is.
You say you got your Chrome bookmarks successfully, but not the passwords from Chrome, and neither from Firefox?
Is there any type of error or message you get when trying the ones that do not work?
Once you get all your bookmarks and passwords into Opera, I recommend signing in and selecting synchronize to add a pass phrase to sync everything. So every new machine or OS you simply install Opera, log on and sync, and bam, all bookmarks and passwords from the server/cloud, using an encryption key only you have (not even Opera knows).
As for now, there's an Opera blog that may help: https://blogs.opera.com/news/2014/12/how-to-import-bookmarks-from-another-browser/