This guide will help you get through the 3rd, 4th, 5th, and 6th night! It got me through, so hopefully this strategy will work for you too! This guide Is fairly short, because the strategy for Night 3 carries over to Night 4, 5, and 6.
So you've made it to Night 3. Freddy begins to move on this night, making all four animatronics active from here on out. Night 3, although can be a bit hectic, isn't too difficult. There IS however, a way to keep Freddy on the stage for the entire night. Freddy's pattern is as follows. He starts at the Show Stage, then he will move to the dining area. Then he will go to the restrooms, then he will go to the kitchen, listen for the Toreador March. Then he will go into the East Hall, then finally the East Hall corner. Now, unlike Bonnie and Chica, Freddy will not appear in your door lights. The East Hall Corner is the farthest he will go before actually attacking you. If you see Freddy in the East Hall Corner, shut the right door immediately! If you fail to do so in time, Freddy will kill you. The way you can tell if Freddy has moved is if you hear him laughing. Every time he laughs he will move to a different camera. This applies to the East Hall Corner as well, if Freddy is there, keep the right door closed and listen for his laughter, that will let you know he is gone. Normally, when Freddy leaves the East Hall Corner, he will reset at the Dining Room. He may sometimes reset at the Restroom or Kitchen, but usually it's the dining area.
There is a way to keep Freddy from moving at all. You know how you frequently check Foxy to keep him in Pirate Cove? Well, do the same thing to Freddy. If you frequently look at the camera Freddy is on, and do it at a consistent rate, Freddy won't actually move at all the entire night. Basically, you want to get into a flow like this, left door light, right door light, Freddy cam, Foxy cam. Rinse, repeat until 6 AM. Don't even bother looking for Bonnie and Chica on the cameras, since they appear at your door light anyway. Looking at Bonnie and Chica on the cameras Is ultimately a waste of power. Only use the camera on Freddy and Foxy. Also, if Freddy does happen to leave the stage, don't panic. Do the same thing you were doing, except check the room Freddy has moved to, instead of the Show Stage.
Alright, Nights 4-6 are pretty much the same exact deal as Night 3. The only difference is that the animatronics get progressively faster, making power conservation a bit trickier. What I usually do at the very beginning of each night, is wait for about 10-15 seconds before doing anything, that way you can conserve at least a little power, so it will hopefully last you through the night. As far as power is concerned, there is no concrete strategy for power conservation, it all depends on how long Bonnie and Chica camp at your doors. I hope this guide helps you survive Nights 3-6.