These three filters serve to help you make decisions about what cars to keep from your holding pool, investigate cars in your garage you may want to unlock for sale or fusion, and generally manage the collection aspect of the game.
The Uniques filter will display in the current view only cars of which you currently own one single copy - this will function the same across 'My Cars' and 'All Cars'. In the Holding Pool it will enable you to filter the cars waiting to be added to just show cars with a single copy already in your garage.
The Duplicate filter will display in the current view only cars of which you currently own multiple copies - in 'All Cars' it will show each model of which you own 2 or more copies in your garage, and in 'My Cars' it will show every copy of each car of which you own 2 or more. In the Holding Pool, it will filter down the cars waiting to be added to just show with 2 or more copies already in your garage.
The Not Owned filter will allow you to display in 'All Cars' only the models you do not currently own in your garage. In the Holding Pool, it will filter the cars waiting to be added to just show those that you do not currently own. This filter does not do anything in 'My Cars'.