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The op-amp is an high-impedance device. The one in the figure is configured as a non-inverting amplifier. You won't get no op-amp whith 3A input current, in fact ANY op-amp is meant to draw as little current as possible. Several kiloohms is just about the minimum input impedance you may find.
It SEEMS to me that Rl may draw something like 3A from the power supply and you want to measure the current that is flowing in Rl, but you need nowhere 3A into the op-amp.
This circuit is measuring the voltage drop at Rs, which is propotional to the current that flows through it an hence through the load (since they are in series). With a large input impedance in the op-amp, no significant current will flow to it so the previous sentence is correct. Yes, the op-amp output voltage will be proportional to current. You just need do add an A/D converter and then feed this to the microcontroller. You can choose a microcontroller with an A/D converter embedded (there are lots of them) which will simplify your work.