After eating a hearty meal they went to see the Buckingham palace. There, Rachel learned that w visitors came to the Buckingham palace that day. If there were x visitors the previous day and y visitors the day before that How many visited the Buckingham palace before that day?
First, we need to find out how many visitors there were two days ago. We know that w visitors came to the Buckingham palace today, and we know that there were x visitors the previous day. This means that there must have been w + x visitors two days ago. 

Now, we need to find out how many visitors there were three days ago. We know that there were w + x visitors two days ago, and we know that there were y visitors the day before that. This means that there must have been w + x + y visitors three days ago. 

This means that the answer (symbolic expression) is: 

w + x + y