I never mention anything related to GND_3_o or PWR_3_o in my code. What are these signals, should I be concerned about these warnings related to them, and if so, how can I fix them?
WARNING:Xst:653 - Signal <GND_3_o> is used but never assigned. This sourceless signal will be automatically connected to value GND.
WARNING:Xst:653 - Signal <PWR_3_o> is used but never assigned. This sourceless signal will be automatically connected to value GND.
XST Warning 653 is a critical warning and should be fixed in your design, otherwise some features won't work as expected.
WARNING:Xst:653 - Signal is used but never assigned. This sourceless signal will be automatically connected to value GND.
GND_3_o sounds like an auto generated signal. Elements in vectors ofter get translated to signal names with underscores.