Is it possible to use an external c# application to access the v3 API endpoints.
I had read that you were working to make it available a year ago.
I know that you can use the old SOAP api but it does not expose everything that I need.
Any information on this would be appreciative
Business doesn't wait for features, so until 3rd party is implemented and depending on your needs you could use liquids module_data and a json output to "build your own" API. I've done this for mobile apps for clients who were using BC that and it worked quite well for their needs. There are limits to how much data you can pull at one time so make sure you understand that.