Answer: Hi!
Let's start, you have 3 boxes and 3 types off eggs inside of each. The fact that they each box is labelled incorrectly means that inside the box labelled Large, only can be the set of small or mixed eggs.
So, let's suppose we open this Box labelled LARGE and inside of it we find the small eggs.
now we have two remaining boxes, the labeled SMALL and the labelled MIXED.
the MIXED can only contain the set of large eggs or the set of small, because you know that is wrong labelled, and also you know that the small eggs are in the first box that you oppened, so here are the large eggs, and now in the last box is the last set of eggs, the mixed one.
So you only will need to open one box to know which eggs are in what box.