Okay this post is nothing but a link, which is this one:

Download Unity 2017.4.9f1 (latest version as of this post (yeah I know, it’s almost 2019 but I guess Unity isn’t big enough to hire a real Linux maintainer)

Seriously, that link should’ve been the first result when you google ‘Unity3D Installer for Linux’, but google thinks you want to read a blog post instead. Typical third rate search engine.