This question has the same background as Getting stuck on the wrong answer, but I thought it merited a separate question. Our four year-old daughter quite frequently knows the answer, or could know if she paid better attention, but because of the way her siblings respond, she either thinks their response is normal or more fun, which causes her to not get as much out of the exercise as she could. How can we help her be her own person and work to her potential instead of copying her siblings' abnormal behavior?
Why do you ask all of them the same question at the same time? That seems to naturally lead to a copying situation. If quizzing them is an important part of your teaching technique, I would suggest asking 3 different questions on the same video so that each child answers independently. Alternatively, ask a simple factual did-you-see-what-happened question to one child, and a more elaborate question to the next: why? what might come next? what else could it be? Encourage them to cue or tip each other, too, so if someone can't remember what colour something was, the others can give clues or otherwise be helpful. If it's a co-operative effort, there's less pressure to be "better than" or "as good at it" compared to the siblings.