Please show me a built up area where you have clear line of sight for 3 miles. 3-500m would be optimistic. You would have 10s to 100s of 15cm drones. They would flit around bins, cars, buildings and through windows. A racing drone can pull 4.5G of acceleration. It can spin that in a fraction of a second.

3G is enough to cover 300m in around 5 seconds. That's also assuming it is going from a dead start. If it can build up speed before entering line of sight, it would be even quicker.

Even worse, they could easily spend 30 seconds to manoeuvre around you. The sensor package drones (cameras, lidar etc) playing peekaboo, to snatch data. By the time they move, they've built a complete 3D map. They know every blind spot, every area the gun can't target. Your gun will go from nothing to shoot, to too many targets in a second or so. Most will just have extra batteries. They exist to draw fire. A few will have payloads designed to target your defences. Others will have payloads aimed at breaking up your situational awareness.

If you engage the micro drones, then your firing arcs will give windows for heavier elements to engage you. If you don't, then the armed micro drones will damage your defences or block your sensors, to create the same effect.