Flattening an array, without using .flatten

gistfile1.txt

a_hash = [[1,2,[3]],4] 

flattenedArray = []
a_hash.each do |element|
	if element.is_a?(Array)
  		element.each do |inner_element|
    		if inner_element.is_a?(Array)
      			inner_element.each do |third_layer_element|
        			flattenedArray.push(third_layer_element)
       			end 
    		else 
      		flattenedArray.push(inner_element)
    		end
  		end
	else flattenedArray.push(element)
	end
end

flattenedArray