Your best bet would be to support the frame with jackstands and then lower the axle enough until you can muscle the spring on. You may even need to disconnect the control arms to gain enough clearance. Once its down have a buddy put their weight on the axle while you push the spring in. If you dont have a pal handy, make sure you have a pry-bar. Watch out for the differential breather tube when lowering the axle. Also, have fun with the upper rear shock mounts. All I can say is have an air chisel at the ready.
Good luck, I just put my 2.5" lift in and it was a bear, so I cant even imagine what 3" would be like.