I would strongly recommend powered extenders though (that get power through an extra molex connector). Many motherboards dont like supplying 3 power hungry cards, particularly not on PCI-E 1x or 4x slots.
Cablesaurus says you only need powered extenders if you're using a high powered card (5970 or 6990, and perhaps 6970 and 5870), but any lower powered cards should be fine, even if you have multiple.
additional tip: if you're not hurried for time, you can buy extenders from Chinese sellers for way less. (around $4-$5)