First, press the ` key directly under ESC. A black bar should open on the bottom of your screen.
There are a couple console commands. Here are a couple fun ones, though.

no-clip 1. press ` || 2. type "ghost" minus the quotes into the console bar.
fly 1. press ` || 2. type "fly" minus the quotes into the console bar. || 3. to disable flying, type "walk" into the console bar, minus the quotes.
size change 1. press ` || 2. type in the word "changesize" || 3. put in the requested size in numbers.

WARNING!
You CANT ALT+F4 out of Neighbor3 for some odd reason (from my experience). Proceed with caution!

Open the console with ` and type in "servertravel Neighbor3" without the quotes. You will be transported into a dark, gloomy, house. It was used as a test for Hello Neighbor Alpha 3, as the name suggests, but was scrapped. It wasn't removed until Alpha 4. It has a fully furnished bedroom, and a fully furnished kitchen. Plus a fully active neighbor on patrol!

As stated before, you can't exit normally out of Neighbor 3, so enter at your own risk.

WARNING!
You CANT ALT+F4 out of Neighbor3 for some odd reason (from my experience). Proceed with caution!

you leave Neighbor 3 by opening console with the ` key, then typing "servertravel level1", minus the quotes. Now, press enter. You should be teleported to the normal world now.