Use SassC in Middleman 3.x. (Also see: https://rubygems.org/gems/middleman3-sassc)

config.rb

require 'middleman-sassc'

Gemfile

gem 'sassc', '~> 1.7.1'
gem 'sassc-rails', '~> 1.1.0'

middleman-sassc.rb

require 'sassc'

# Monkey patch Sprockets to use SassC
module Sprockets::Sass
  class SassTemplate
    def evaluate(context, locals, &block)
      # empty file?
      if data.empty?
        @output = ''
      end

      @output ||= begin
        @context = context
        options = sass_options
        options = options.merge({
          load_paths: context.environment.paths,
          importer: ::SassC::Rails::Importer,
          sprockets: {
            context: context,
            environment: context.environment
          }
        })
        ::SassC::Engine.new(data, options).render
      end
    end
  end
end

# Middleman also tries to call the Sass engine sometimes
module Middleman::Renderers::Sass
  class SassPlusCSSFilenameTemplate
    def evaluate(context, _)
      @context ||= context
      sprockets = context.sprockets
      @sprockets_context ||= sprockets.context_class.new(sprockets, File.basename(file),
        Pathname.new(file))
      @engine = ::SassC::Engine.new(data, sass_options.merge({
        load_paths: context.sprockets.paths,
        importer: ::SassC::Rails::Importer,
        sprockets: {
          context: @sprockets_context,
          environment: sprockets
        }
      }))
      @engine.render
    end
  end
end