In some cultures, yes, many GNC gay people would be considered part of a 'third gender', and this was also the case in a lot of the West before modern separatist movements came into the picture. There's still an overlap today between he/him lesbians, stone butch, drag kings, and transmasc communities, just like there are overlaps between effeminate she/her gays, drag queens, and transfemme communities.

Also, non-binary people can physically transition through various surgeries and hormones, in addition to social transitioning. It's often very difficult because much of western society doesn't recognize their existence, so NB people are often forced to choose between the 'socially acceptable gender' of either male or female to live as legally, which can in turn lead to major dysphoria.