A 3yo probably can't pull a trigger without using 2 fingers, even a light trigger. Then there is the issue that there is probably not a gun built that will even remotely "fit" a 3yo such that they can shoulder it and even reach the trigger much less shoulder it, reach the trigger, and see through the scope.

My son was 4 or 5 when he started shooting the smallest daisy made "lil buck" which is several inches shorter than a red ryder. That thing is lit a toy even compared to an AR with the stock collapsed.