My ruby koans triangle implementation. Is there a "rubyer" way to write this?

rubykoans triangle.rb

def guard(a, b, c)
  if a < 0 || b < 0 || c < 0
    raise TriangleError, "a triangle should not have a side with a negative value." 
  end
  if a == 0 && b == 0 && c == 0
    raise TriangleError, "A triangle should not have all its side equal to 0." 
  end
  if (a + b) <= c || (b + c) <= a || (a + c) <= b
    raise TriangleError, "Any two sides of a triangle should add up to more than the third side." 
  end
end

def triangle(a, b, c)
    guard a, b, c
    return :equilateral if a == b && b == c
    return :isosceles if a == b || b == c || a == c
    :scalene
end

# Error class used in part 2.  No need to change this code.
class TriangleError < StandardError
end