Open Lightroom CC. Choose File->Open Catalog and then point to each of the Lightroom 3 catalogs, one-by-one.
Adobe Chat Support told me LRCC won't update LR3.
If that's what they said, they are 100% wrong.
First let me ask, Where are all your images? On the old Mac computer?
LR doesn't copy or move images into its own folder structure. It leaves them where ever you placed them on your hard drive. So you first need to copy all your images to an external drive and or copy them to your new Mac. You do that also with the complete LR 3 catalog folder including the Previews folder.
Once you have them all copied to the new Mac when you open LR CC on the new Mac it will find the LR 3 you copied over, Make a Copy of the catalog and Convert that copy to a LR CC catalog file.
Thanks. LR "knows" where each image file is and if the files are not moved within LR you have to go find the file. Because the hard drive on both computers is named "Macintosh HD" I assume that LR will think the files on the new laptop are in the same place as on the old one. Otherwise I'm going to spend a lot of time "finding" them. Files on external drives should be no problem as the file location doesn't change regardless of which computer the drive is connected to. Hope I'm right.
Thanks. I'm going to do this and will let you know. First level support folk unfortunately often provide wrong information.