From lightroom 6 to 3.3

Lightroom 3.3 works with a catalog, but it's in the cloud and you don't have immediate access to it like you do in Lightroom 6. You can't back it up, and you only have the one catalog. In order to get the images you have already done into that catalog you would have to install the trial version of Lightroom Classic long enough to convert  your existing catalog to the updated format. Then it would be necessary to migrate that catalog to Lightroom 3.3. Lightroom is organized somewhat differently than you are accustomed to. Your images are stored in the cloud. If you choose the right plan you will have 1 TB of cloud storage to begin with. If you fill that up then it will be necessary for you to purchase more space from Adobe.