on MacBook air, mac os 11.3, i am unable to load pics to Photos 6.0. Photos came from an older mac with Photos 3.0

As yer man says.

If you have copied the photos app to the USB drive, you have not copied the photos, just the program. (This is like copying the pages application without copying the pages document files).

You need to copy the library (which contains the actual photos) - usually called "photos library.photoslibrary" in the pictures folder. But don't try to copy it on a thumbdrive. Either copy it via a time machine backup, or use an external drive formatted APFS or MacOS extended. Thumbdrives are not suitable for storing photo libraries.

When you have copied the old library into your new pictures folder, you can open it by starting the photos app on your new mac while holding the option key. You can then select the old library to be opened, and the format of the library will be upgraded for Photos 6.

What exactly did you copy to the drive?

A Photos installation has two parts: the application, which lives in the Applications folder, and the Photos Library, which (by default) lives in the Pictures Folder.

It sounds from this message:

"You can't use this version of the application "Photos" with this version of macOS." Below this message in small print, it says, "You have "Photos" 3.0.

that you're trying to open the application and not the Library.

Did you have Time Machine enabled for your old MBA? If so you can restore the library from it.

How are you trying?

The usual route is to open the v3 library with v6 Photos.