Current amplification for Raspberry PI4 powering
The common 9 volt rectangular batteries cannot supply 3 Amps - they are designed for low current applications - I think even 100 mA is a bit much for them.
The LM7805 is a linear regulator and will waste over a third of the power you draw from the battery as heat.
Four AA cells and a DC-DC converter (switching power supply) may work, but 3 Amps is really too much for an AA cell - C or D cells would be better.