I am also concerned about video card capability with Premiere Pro 2015.3.
My card (GTX 670/4gb) was current only three years old but this era seems be referred to as "legacy" in the Premiere forum. The card falls in the middle of the age of others on the supported cards list in system requirements and works well with 2015.2.
Will this card be compatible in with 2015.3?
Also, though the view though the lens of forum discussions can be distorted, is the legacy card concept designed into 2015.3 or am I getting the wrong impression from the posts?
I have the GTX 680 in my Mac Pro (mid 2012) and have had many issues recently. All of them were CUDA related. I can get by with Open CL right now but was recently informed by an Adobe rep to expect trouble in the future as this is a legacy card (even though it is on Adobe's list of supported cards). He advised me to upgrade soon. Don't trust that list of supported cards.