Just to make 100% sure that “I have the same problem” is indeed the same problem 
- your computer runs Windows 10?
- the installer does not detect that you have a Fairphone (as opposed to having detected the FP3 but then being unable to do the actual installation)?
Helpful in any case would be to upload the logs of the installer.
You can find the log(s) depending on your operating system in different locations, see Howto troubleshoot issues with the easy-installer
The log might be too large to upload it to the forum directly. In that case you can try one of the the various pastebin services, like https://pastebin.com/, https://paste.c-net.org/, https://paste.ubuntu.com/