I'm not sure your PSU draws enough power to plug more than 2 graphic cards on it
What do you think ? Are you trying to plug 3*GPU to a 600W PSU ?
I have 2 600w PSUs. One is running 2 GPUs and the system itself and the other is running just 2 GPUs. I've tested the PSUs to make sure they're not the issue and I have ruled them out. Both work just fine running 2 GPUs with no issues.