Customer+Account+Invoice Date =  'MyCustomer 1234 {01/02/2009}'

will be faster than

 Customer = 'MyCustomer'
AND Account = 1234
AND Invoice Date = {01/02/2009}

but not by a lot

But you will in any case be having additional indexes on Customer, Account and Date. So having another index will increase your data saving time and also the size of the indexes. You then have to see the frequency you require for using the Customer+Account+Invoice Date and then decide if you really need the extra index.

In VFP the order in which you use

Customer = 'MyCustomer'
AND Account = 1234
AND Invoice Date = {01/02/2009}

can also change the time required. So

Customer = 'MyCustomer'
AND Account = 1234
AND Invoice Date = {01/02/2009}

is not the same in time taken as

Account = 1234
AND Customer = 'MyCustomer'
AND Invoice Date = {01/02/2009}

Regards
Tushar