Something like this?

;WITH family1 AS (

SELECT DISTINCT InvoiceID

FROM invoiceDetails

WHERE ItemCode LIKE 'PMM1%'

), family2 AS (

SELECT DISTINCT InvoiceID

FROM invoiceDetails

WHERE ItemCode LIKE 'PMM2%'

), family3 AS (

SELECT DISTINCT InvoiceID

FROM invoiceDetails

WHERE ItemCode LIKE 'PMM3%'

)

SELECT SalesRepID, COUNT(*) FROM Invoices i

INNER JOIN family1 f1 ON f1.invoiceID = i.InvoiceID

INNER JOIN family2 f2 ON f2.InvoiceID = i.InvoiceID

INNER JOIN family3 f3 ON f3.InvoiceID = i.InvoiceID

GROUP BY SalesRepID