The original AW will be only one year behind when wOS3 is released so it'll most certainly be supported. Apple doesn't obsolete any of their products after only one year.
It's not like having iOS 9 on the ninth-generation iPhone 6s/6s Plus where the numbers match in term of generation.
I think it will support several wOS Versions. I think it will come to a Feature breakdown. Something like: yeah, our Hardware on our first Watch cant handle it effectively, so we will not offer you the wOS4 special Feature XY.