While wintun support is not yet added to openvpn3 master, I think it would be beneficial to share my own branch.

https://github.com/lstipakov/openvpn3/tree/feature/wintun-wip#building-the-openvpn-3-client-on-windows  

See instructions in README on how to build openvpn3 library and test client on Windows.

Note that only "topology subnet" is supported (see https://community.openvpn.net/openvpn/wiki/Topology#Topologysubnet).

I have included test client and wintun driver binaries if someone would like to try it out (Tested on Windows 10 and Windows Server 2016):

1. Since wintun driver is not signed, you need to enable loading of test signed code. For that, run

> bcdedit /set testsigning

in admin command prompt and restart Windows.

2. Download and unzip wintun driver https://github.com/lstipakov/openvpn3/blob/feature/wintun-wip/wintun.7z

3. Open Device Manager (Control Panel\System and Security\System\Device Manager). 

Right click on tree root, select "Add legacy hardware" -> "Next" -> ".. hardware that I manually select from a list" -> 

"Next" -> "Show all devices" -> "Have Disk..." -> Locate wintun.inf unpacked at step 2 -> select "Wintun Userspace Tunnel" ->

"Next" -> "Install this software anyway" (since driver is not signed).

4. Make sure you see "Wintun Userspace Tunnel" under Device Manager -> Network adapters.

5. Download cli_wintun.exe from https://github.com/lstipakov/openvpn3/blob/feature/wintun-wip/cli_wintun.exe

6. Open command prompt as an administrator and run:

> C:\Users\Administrator\Downloads>cli_wintun.exe <your_vpn_profile.ovpn>

7. Enjoy!