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 day before yesterday, there were y visitors. The day before that, there were x visitors. So, two days ago, there were x + y visitors. 

Yesterday, there were w visitors. So, three days ago, there were x + y + w visitors.