You can install Octostrap3 like any other Octopress theme. Here is how to do it in a few steps:
% git clone https://github.com/imathis/octopress.git
Installing is straightforward, just clone the theme and run
% git clone https://github.com/kAworu/octostrap3.git .themes/octostrap3 % rake "install[octostrap3]"
And that’s it ! You can start blogging if the default theme suits you or continue this setup guid for more awesome customizations :)