Sum of the Last Three Orders

For each order return the sum of that order and the two orders before it for the same customer (fewer when not available), ordered by id.

Table schema

Orders(id INT PRIMARY KEY, customer_id INT, amount INT)

Sample data

id | customer_id | amount
1 | 1 | 10
2 | 1 | 20
3 | 1 | 30
4 | 1 | 40