Free with more than 2 algebra

I am trying to define a `Free` monad similar to http://typelevel.org/cats/tut/freemonad.html using 3 instead of 2 algebras. However, I am having issues to get `Inject` automatically derived. Here is the best solution I could come up with but I would like to get rid of the implicit `inject3a`, `inject3b` and `inject3c`

https://gist.github.com/julien-truffaut/78cc00b9ff5d4bb234e3
