Hi there,
What are the model numbers of these drives, and are they 3 1/2" drives or 2 1/2" drives? Also what kind of external exclosures do you have them in?
The USB ports supply 500ma of current at 5V, so it's not a lot of power turn the disc spindles and run the electronics of external drives. The USB 3.0 ports supply 800 ma.
While one ext HDD is running, plug a USB flash drive into one of the other ports you are using, to see if the OS recognizes it, which takes essentially min power to run. If so, it's probably a power issue. The size of PSU has no bearing on the current supplied by each USB port.
The other thing you might do is go to the Device Manager, and double click on each of the USB hubs separately to bring up the USB hub properties. Click on the Power tab and it will tell you the current used by any devices connected to that hub.