All of them can be found at nyaa.si. Book five is by an official company, but 6 and 7 are just by a lone fansubber who's tired of waiting. Thank you, [avarice]. I too was tired of waiting.
Hai to Gensou may be the best story ever, depending on what happens in its subsequent volumes, so three books from this series is no light thing. It may be the best moment of 2018.