Count Uniques with 2 criterias

Hi

I have a spreadsheet with the following columns in Sheet1

Order Taken BY
Order Line ID (This can occur multiple times as the same number if customer order say 4 order lines)
ORDER Source

What I want to be able to do is create a report which shows the following in Sheet2

I want to enter the username into A3 ie: Ian
I want the Order Source in B3 ie: Telephone
Then in D3 I want to know hwo many unique orders that sales person took and I am stuck creating a formula to count the order lines and see if they are unique and then return the result. I can get it to tell me how many order lines there are but I need it to be unique as for one sales person it is showing 5 orders lines the same so he has completed 1 Telephone Order.

Any help appreciated.

Thanks

Roy