Your 3.3V Li-PO is almost dead. It is 4.2V when fully charged and is 3.7V when half-charged. it should be disconnected and re-charged when it is 3.2V.
Are you drawing 5A from the 3.3V Li-PO? That is 5A x 3.3V= 16.5W. Maybe 1.6W is wasted on heat then the 5V output will be 15.5W/5V= 3.1A but a Li-PO that is almost dead cannot produce the original 5A.