In order to make an estimated tax payment, the IRS requires that all taxpayers use the 1040 Estimated tax vouchers (Form 1040 ES)
for the appropriate quarter.
Hence, in your case, you would use the 3rd quarter 1040 ES Voucher that needs to be mailed by September 15, 2008.
Click on the IRS link below in order to generate the 1040 ES voucher's for all the 4 quarters. In addition, carefully read the instructions so that you can accurately prepare the appropriate 1040 ES vouchers. http://www.irs.gov/pub/irs-pdf/f1040es.pdf