well, the only real answer is to "do wut those muscles do," so just perform the mvnts that those muscles produce when they contract (against resistance, of course). Therefore, to work the Trap III (mid trap) you have to Retract the scapular (pinch it back). You do this (whether intentionally or not) anytime u do a high row or reverse fly. But if u wanna isolate it, just let ur arm(s) out straight in front and pinch back. Thats all u can do. U can do it in various body postions and with various forms of resistance, but its still same mvnt. EXAMPLES: position urself on a a bench (one hand and one knee on bench) other foot on floor, grab a dumbell (essentially the position youd be in for a unilateral bent over low row. But instead of "rowing" let the arm hang/dangle and JUST pinch back with shoulder blade. Although its not 100% necesary to do the other arm (since the mid trap connects the two shoulder blades, so its not like theres a muscle "on the other side" I guess it culdnt hurt to swap arms/sides. ALSO: sit on ground at multi cable crossover machine (legs facing cable) cable at chest level, attach straight curl bar (palms up will giv u more ROM) and with arms extended fwd, just pinch shoulder blades back.

TRAP IV (and pec minor): get on Dip bar, get in top position (lock arms) sink down, and then contract up (just using the scapulae). OR: sit on floor, legs extended, feet up on a bench (kinda like ur doin a two-bench dip).....but without one of the benches, haha. Instead of dipping, keep arms locked, sink dwn, thrust/contract up. might wanna get a cpl hex-head dumbells to put on flr for ur hands. Youll be able to sink dwn further, and also lessen torque on wrists. BEST: is stand under multi cable: right up next to the column (like ur married to it). with handle positioned as high as possible, grab and get it down by ur waist (arm locked) downward. let the cable pull ur shoulder blade up, then thrust/contract downward. best if u get ur hand on other side of handle (like with the cable between u and ur hand/handle) with the cable literally rubbin against front shoulder. its a subtle position change but will allow u to "get on top" of the resistance with the best angle.

not sure if this makes sense hope it helps