T-SQL (SS2K8) :: Return A Count Per Unique Check In Invoice Table?

We have a table that has customers invoices and payment records. In some cases a customer has 10 lines with 10 different invoice numbers but may have paid 2 or more invoices with one check. I need to know how many unique payments were made per customer.

Cust# Inv# Chk#
1 109 101
1 110 101
1 111 102
3 112 10003
2 113 799
2 114 800
1 115 103
3 116 10009
2 117 799
1 118 103

So I need the statement to update the customer table with the annual payments

Customer Table
Cust# Payments
1 3
2 2
3 2

I get close but just not getting it to sort itself out.