The code indicates a bad DSU in mag 3.......that doesn't necessarily mean that you don't have a bad LSU.
Check the cable from LSU/DSU and the outputs on the power boards first. Then, if it were me, I'd either disconnect the DSU in mag3 and try to load the switch again, or I'd pull all the device boards (except LSU) in mag3 and try a load.
What event caused the LIM to drop in the first place?
Good luck,
Dave Strang