triangle.rb

class Triangle
  attr_reader :a
  attr_reader :b
  attr_reader :c

  def initialize(a, b, c)
    @a, @b, @c = a, b, c
  end
  
  def type
    return :impossible if sides.any? { |side| side == 0 }
    
    case equal_sides_count
    when 3 then :equilateral
    when 2 then :isosceles
    else :irregular
    end
  end
  
  private
  
  def sides
    [a, b, c]
  end
  
  def equal_sides_count
    sides.inject(Hash.new(0)) {|counts, side| counts[side] += 1; counts }.values.first
  end
end

triangle_spec.rb

describe Triangle do
  describe "#type" do
    subject { triangle.type }

    context "for impossible triangle" do
      let(:triangle) { Triangle.new(1, 0, 1) }
      
      it { is_expected.to be :impossible }
    end
    
    context "for equilateral triangle" do
      let(:triangle) { Triangle.new(1, 1, 1) }
      
      it { is_expected.to be :equilateral }
    end
    
    context "for isosceles triangle" do
      let(:triangle) { Triangle.new(1, 2, 1) }
      
      it { is_expected.to be :isosceles }
    end
    
    context "for irregular triangle" do
      let(:triangle) { Triangle.new(1, 2, 3) }
      
      it { is_expected.to be :irregular }
    end
  end
end