As Rebar3 is free, open source and written in Erlang, it's possible to simply clone and build it from the source code.
$ git clone https://github.com/erlang/rebar3.git
$ cd rebar3
$ ./bootstrap
This will create the rebar3 script, which you can put on your PATH or link to /usr/local/bin
as explained in the section "Installing Rebar3" above.