JavaScript - 为什么我的第三张幻灯片不显示？

When I click the prev and next sign, only 1.jpg and 2.jpg img works, 3.jpg never shows up (I can only navigate between 1.jpg and 2.jpg). Console shows no error. I have been look at it for hours, I seriously don't think my code has anything wrong.When I click the prev and next sign, only 1.jpg