The reason I made this one is for my sand city in the desert, look at the 1st screenshot and you can find it out!
Actually it does not has to be 3x5, may be 3x1, 3x2, 3x3...3xn if you know how to extend it; you can even make a wall out of it. I'm not going to spam because it is really simple as it shows above: screenshot 2 is the naked version and screenshot 3 is the 3x3 grass version. For a 3x3 gate, the core mechanism only takes 4x5x5 blocks. You can use any gravity-based block like concrete powder or gravel for the gate.
Here I'm not posting a pure redstone version, because some people hate naked redstone so I put a covered theme of a sand gate here (cover picture).
And at last, if you hate the input position, just dig a bit and tweak around, it is really simple so I'm sure you can do this! I just made something work right after your paste and save everyone's time for enjoying the game :).
How to import by structure blocks (in vanilla MC):
~ Thanks to BSL shaders and Soartex resource pack for the amazing screenshots.
~ Thank you for your supports!