3x3

function digIt()
 while turtle.detect() do
 turtle.dig()
 os.sleep(0.5)
end
 turtle.forward()
 while turtle.detectDown() or turtle.detectUp() do
  turtle.digUp()
  turtle.digDown()
 end
end
function placeTorch()
 turtle.select(16)
 turtle.placeUp()
end

local run = 0
term.write("Tunnel length:")
run = read()
for i = 1, run do
 k = i - 1
 j = k % 4
 digIt()
 turtle.turnLeft()
 digIt()
 turtle.turnRight()
 turtle.turnRight()
 turtle.foward()
 digIt()
 turtle.back()
 turtle.turnLeft()
 if i == 2 or j == 1 then
  turtle.back()
  turtle.turnRight()
  turtle.forward()
  placeTorch()
  turtle.back()
  turtle.back()
  placeTorch()
  turtle.forward()
  turtle.turnLeft()
  turtle.forward()
 end
end