I have an external drive with thousands of photos processed in a version of iPhoto (can't recall the exact version). Can these photo files be read by Photos 3.0? Thanks!

Try the following:

• Connect the external drive to your new mac.
• Launch Photos on your new Mac while holding down the options key ⌥ until you are seeing the dialog to select a library.
• Select the iPhoto Library on the external volume.

Photos should now create a new Photos Library in the same folder as the iPhoto Library and you should be able to access the iPhoto photos in photos.

This requires, that the external drive is compatible with Photos - check the file system format as described here: Move your Photos library to save space on your Mac - Apple Support. The format of the file system should be MacOS Extended (Journaled) and you sholud be able to set the "Ignore ownership on this volume" flag.

If your photos are in an older iPhoto Library, you can open the iPhoto Library in Photos 3.0 to create a new Photos Library from the iPhoto Library. Photos can directly open Libraries from iPhoto '11, version 9.xx, or iPhoto '09, version 8.x.x. If the iPhoto Library has been created with an older iPhotoversion (iPhoto 7.x.x or older), you have to prepare the library with iPhoto Library upgrader tool, before Photos can work with it. This has to be done, while you are still having a Mac with macOS 10.14 or older. The iPhoto Library upgrader cannot be used on the more recent 64-bit only system versions. The download link for the iPhoto Library upgrader is here:

iPhoto '11: About the Library Upgrader

Are they in iPhoto Libraries? OR just in folders in the Finder?

Either way, yes.

Good Luck! 😊