OK. 3 plates is heavy for shrugs? ... 4 kinda and 5 yes. If I do them with a slight forward lean, I feel some trap recruitment. I've never tried a trap bar, which I suspect would be lots of fun. Behind the back shrugs are for me totally worthless. Cleans are good for me, as are DB snatches. Then again, I have never focused on traps and shrugs. Separating both of my shoulders over the years has not helped me a damn bit.
I'm considering a day to target weaknesses - traps, delts, tris, calves, grip, ... wait ... f*ck ... that's everything.