During the install process of LR 6 you will be prompted if you wish to create a updated version of your Catalog from LR 3.
If you agree then a copy of the Catalog will be made which will be compatible with LR 6. In addition the "previews files" will be made compatible with version 6.
I am not sure what you mean by "flip over"?
The Catalog file does not actually contain the photos its just a databased file that contains the info as to where the files are and the work you have done with them in Lightroom.
Your original files will remain just where you have them saved and will do no "flipping".
In addition to my earlier post you do not have to remove/uninstall LR 3 before installing LR 6, they will both reside independently on the same computer.
Converting Catalog means that all the work, preferences etc will be adopted by LR 6.
When you are comfortable with the new installation and thing are running as you expect then you can decide to uninstall LR 3.
Bought Lightroom 6 and it says we have to upgrade from Mac OS6.8 to 6.10 so we are going to buy Mountain Lion from Apple......is that all we will need?
I have a headache........thanks!
The current Mac OS is 10.11.something. Can you get that to run on your computer?