guiminer connects to stratum.bitcoin.cz:3333 ok, but it can start all 3 gpus, only 1 does any work.
With guiminer and multiple cards you need to create another miner for each card. Just go to file > new miner and setup another one. Then use the device dropdown list to select the next GPU. Create one for each GPU and you should be fine. If you don't already know about it you can select view > show summary and see all of the miners at once.
Yeah, that's what I was gonna say. It sounded like he was using the same gpu for all three miners.
Make sure the device drop down selection is different for each miner...