If I had only 3" clearance with anything but coilovers, I would consider that too close, 3" clearance with a 3" or 4" coilover would be fine. With either leaf or coil springs on the rear, I'd do a C notch in the frame to pick up another 1 1/2" of clearance, install a snubber to keep it from bottoming out on some really huge bumps, and call it good...