I found these as well but gave up on trying to find the third after a couple of hours, then realized you cannot go back to it after you complete the mission there. definitely upload once you find the third one! I'm probably going to have a good facepalm moment once it's found lol
haha yeah its a pain you cant go back after. Thanks to @damo85 I have found the last button: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-5WVNqcPUB6Y.html