The 2 murderers:

In the FNaCoso 2 minigames, we see two people saying stuff like "Oh no our plan almost failed broda, I had to take the kid out of there broda :O".

I think these people are working together, one being an actual murderer, and the other maybe just an employee, or who knows, a murderer dressing up as an employee! :O

Five Nights at Coso 3:

In the minigames we see a Coso model we didn't see before, and if we look closely it looks a bit similar to Old Coso, and even more similar to that Coso model in the teaser "Our magic world".

The minigames possibly happen at the FNaCoso 3 location, here's why:

1. The building:

   The building shows a different layout from the first 2 games, and the only stage which has one animatronic in the series is Cosonet's.

2. The animatronic:

   The animatronic in the stage is similar to Old Coso, possibly his fixed version, in an older location.

3. The teasers:

   The teasers are about a game we didn't see before.

Now the final question is: If FNaCoso 3 happens before the first game, then why did Cosonet appear with that aspect in the teaser?

Coso and Cosa were created to kill, and they already did it:

In the minigame with the 2 murderers talking, they say that they will need 2 new characters (Coso and Cosa) to kill more children.

In the FNaCoso 2 teaser "Everyone needs a second chance" we see Coso and Cosa and what appears to be ghosts of two children that they killed. Interesting...

These are my theories hope you like them and remember to like this post so MidaGames will feature it! :D