Unlike previous Five Nights games, Five Nights at Freddy's 3 has multiple endings. There is a bad ending, where you beat the game and the animatronics continue being haunted. And there is a good ending, where you put the spirits of the dead children to rest.
You want to put the children to rest, so their souls stop murdering unsuspecting security guards, right?
Getting the good ending is not an easy process—it's almost on the level of P.T. puzzles. But, PrettyGrumpyBear developed a very helpful video that walks you through all the steps. You can watch it, or follow along with written instructions, which I've provided below (along with screenshots and GIFs!)
First, you'll need to get through Night 1. That should be a breeze—there's not much to do on that night. Then, on Night 2, the real process begins.
Night 2 - Go to Camera 8, and click on the Ballon Boy poster on the left—right where the cursor is on this image:
Once you do, you should end up at this mini-game:
Get all the balloons, but don't go through the exit that appears. Instead, go to the top left, and jump out of the blue box, like so:
You'll hit a bottom that looks glitchy. Walk to the right, past the room with the sad crying kids
And you'll eventually walk into the room with the last balloon:
Grab it, you'll end up back at the original game. Here, click on camera 7. On the arcade machine, click on the buttons in this specific order—top left, bottom left, top right, bottom right. Thanks to FNAF's 3 love of static, it's a bit difficult to see, but...
If done right, Mangle's mini-game should start up:
You have to collect your endoskeleton pieces without the the character in the green shirt touching you. Once you do this, ignore the exit once more and jump out of the purple box.
You'll fall down glitch land once more, to a giant horrifying crying kid. Ignore the kid and walk to the left. On the next screen, make your way up the balloons until you reach a cake.
This should bring you back to the original game.
Here, go back to Balloon Boy's mini game on camera 8, by clicking on his poster. Collect all the balloons, and then jump out of the box on the left side. You should fall down and hit a balloon, like so:
Make your way to the right side, where you'll find a crying kid. You'll be able to comfort the kid with cake!
Now beat night 2, and start night 3. You'll need to click on the cupcake on four different cameras: 2, 3, 4, and 6. It's a bit tough to see where the cupcake is, but here's a GIF that should hopefully help you out:
If done right, Chica's mini-game should start up:
Go to the left. On this new screen, ignore the cake, and jump down to the next screen. There, you should see some crying kids. Ignore them too. Hop over the green square on the left, like so:
Go up to the kid, and give him cake.
Now beat night 3, and go into night 4. In your office, there are a few panels you can click. You need to click them in this specific order: 395248.
That should bring you to a new mini game:
Okay, this one is complicated. If you want to watch it in action, skip to the 4:07 mark in PrettyGrumpyBear's video. Otherwise, here are the written instructions.
If you touch a kid, the game will glitch out, and Freddy will fall down over and over again. Don't worry, that's supposed to happen. After a bit of falling, you'll finally hit a bottom. It'll look exactly like the first screen with the kids—you're just finally on solid ground again. Here, walk to the right twice, until you get to an exit screen, like so:
Don't go through that door. Instead, immediately jump up on this screen—you'll start glitching up this time. Before you hit the top of this first screen, jump to the right. This next screen is, once again, the same one with kids and Springtrap. Jump to the top of this new box, and walk to the right. Before you hit the edge on the left, jump up to the screen above.
This screen is, once again, the one with three children—but you are actually someplace new! This time, Freddy should glitch into the room. Get to the right edge of this box, and jump up. You should start glitching up once more. Once at the top of this new box, walk to the left, and jump up before you hit the edge. You'll go into another screen with the three kids. Inside the box, go to the left edge, and jump up. You'll get on top of the box.
Walk to the right edge of the box, and jump over to the right. If you pull all of this off, you'll find the crying kid who decided to hide inside of fucked up glitch land:
Give him cake. Yes, even though this one is a butt.
Now beat night 4, and go into night five. In your office, on the right, there's a plushie. Click on it. It's a bit hard to see, but, it's right at the cursor here:
A Shadow Bonnie mini-game should start up.
Press the "S" key, and you'll start glitching out. Press it repeatedly, until you get to Ballon Boy's mini game:
You'll want to jump through the wall on the left side. Outside the box, press "S" until you see a small crying kid:
Though you are a horrendous, not-comforting version of Bonnie, you'll make the child happy with some cake. Because cake is so powerful, it can quell the sadness of ghost children, don't you know?
Once you do that, you should end up at the main game once more. Click on camera 3. Here, click on the marionette picture on the right side—see where the cursor is?
This should start up a new mini game:
As you walk to the right side, you'll see the kids you've given cake to thus far. Eventually you'll get to another screen where there's a crying kid. Just keep walking to the right, until you put another cake up. This should cheer the last kid up. The kids should all disappear, and the masks will fall down, like this:
Woo! Almost done. Now you just need to beat night five, and you should automatically be taken to a mini game where you have to push Purple Man, the murderer, into the suit:
The suit will kill Purple Man, who was the worst:
And finally, you'll hit the game over screen. Unlike the bad ending, you'll note that the animatronic heads aren't lit up. The going theory is that, by doing aaaaall the stuff in the mini-games, you're putting the children to rest—so the animatronics stop being haunted.
This is also why people are theorizing that FNAF3 might be the last one in the series—if the suits aren't haunted, they can't murder anyone anymore!
I just want to know how in the world people figured this stuff out. Amazing.