Customer rebate accrual calculation collectively for a group of customers

We have defined a customer volume program with the following Tiers
0-500k - 1% Rebate

500-1MN - 2% Rebate

1MN+ - 3% Rebate

We have added three customers in that program and we have a requirement where the customer volume program should look for collective sales of all three customers and then apply the rebate. For e.g. If the sales for each customer is as below:

Customer A has a sale of 200K

Customer B has a sale of 250K

Customer C has a sale of 50K

In the above scenario, the system should sum up the sales of all three customers and calculate the 1% rebate accrual as the total sale is for $500K.