The cheapest and easiest way would be to take a piece of 3" 16ga tubing
then this:
http://vibrantperformance.com/catalog/product_info.php?cPath=1022_1036_1073_1141_1170&products_id=724&osCsid=1a243dc8c1d67e9fe39a2c2e014da671
(since it’s not a boosted application, you could probably get away with a 5 dollar one from pep boys/ebay)
then a short piece of 2.5 16ga tube.
if you really want to have a conical METAL stepdown, then it gets slightly more difficult. (though not terribly…)
easy if you buy this:
http://vibrantperformance.com/catalog/product_info.php?cPath=1022_1035_1211&products_id=1078&osCsid=1a243dc8c1d67e9fe39a2c2e014da671