I just bought a western digital 500 gb external hard drive with only one usb port and no firewire. I have two other external hard drives with firewire and usb ports. I need to transfer data from the two old external hard drives to the new western digital external hard drive. Could there be any problems with this now or latter down the road?

There can always be problems, such is the nature of computing. However, as long as your computers always have at least one USB port and sufficient interim storage for the source drives, I don't see what you are worried about.

As long as the drives are recognized by the system, then the interface is immaterial.