>>479のPointを使って
class My
　attr_reader :position
　def initialize ; @position = Point.new ; end
　def position=(xyz)
　　@position.x,@position.y,@position.z = xyz # 要引数チェック
　end
end

my = My.new
my.position = [0,0,0]
pos = my.position
my.position.x = pos.x + 1