If you need long cables to the U3s, it's better to supply it with 9-12V and have a local 7805 (with caps on both sides) that regulates the voltage close to the boards. Otherwise you will get drops in voltage and all sorts of fun kickbacks when connecting to a distant 5V supply, perhaps mixed with other consumers as well. Microprocessors usually don't like unstable power supply systems.
Daniel Ekman SA2KNG