I'm not sure I understand the
mhA units in your question.
Regardless, your question asks about the source of the low voltage warnings, so we'll focus on that:
The scope traces in the other answer could explain the under-voltage warnings. Here's how that works:
On the RPi 4 (and 3B+) low voltage warnings are triggered by the PMIC (Power Management IC) - p/n MxL7704. An overview of the MxL7704 is available in another answer here if you're interested in those details, but the part relevant to your question is this: The MxL7704 PMIC monitors the input voltage to the RPi, and whenever that voltage falls below 4.63 volts, a Low Voltage Warning is issued to the system.
Why does the voltage fall below 4.63V?
There are two or three possible causes:
The current that flows through your power cord to the RPi creates a voltage drop while passing through the copper conductors. The amount of the voltage drop is given by Ohm's Law:
V = I * R. In other words, the more current that flows, the greater the voltage drop. The current spikes shown in the oscilloscope traces also represent an increased voltage drop in the power cord, and therefore a lower voltage is being delivered to the MxL7704 PMIC.
Raspberry power supply(5.1V,3A) you are using does not regulate the voltage at the input pin of the MxL7704 PMIC. The voltage regulator is housed in the plastic shell of the "wall wart" you plug into your mains outlet. And so, you might imagine that the voltage regulator is saying to the RPi, "Don't tell me the voltage is too low - it was 5.1V when I sent it to you. Take your complaint to the power cord."
It's also true that voltage regulators are not perfect. That is to say that a power supply rated at 5.1V at 3A, will very rarely maintain the exact same voltage across the entire range of its load current capacity. And there will be manufacturing variances, temperature will have an effect, etc, etc. The sharp transients shown in the oscilloscope traces may also be beyond the voltage regulator's transient response capabilities. If the oscilloscope readings in the other answer had measured the input voltage at the RPi, they would be more specific to your question - but they are relevant nevertheless, as they indicate a challenging load that the RPi supply may not be capable of handling very well..
The RPi designers have been walking a tightrope with respect to their power supply design. The input connector they have chosen is the USB-C (for the Pi 4), and the USB-C connector specification imposes physical limits on the wire gauge (diameter) that may be used. Three amps is quite a lot of current for a 22-24 AWG wire, and longer power cords increase the voltage drop.
Available remedies include using the shortest power cord possible, and powering your peripherals through a separate supply. Let us know if you have further questions.