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?
The number of visitors who came to the Buckingham Palace on the day before Rachel's visit is x. The number of visitors who came to the Buckingham Palace two days before Rachel's visit is y. This means that the number of visitors who came to the Buckingham Palace three days before Rachel's visit is x + y. 

Therefore, the answer is: 

x + y