I haven't watched the second half of last season due to events in Libya and what not, so i might not do Rossi much justice as last season was his best season. But i've watched enough of La Liga before i think to form judgment on what type of players these 3 are.
IMO you can't really compare Rossi to the other two, the other two are main strikers while Rossi is the more creative, more of a second striker. Soldado actually plays as a lone striker for Valencia. If you ask me, Negredo has been the best among those 3 in the past few years. He's the best finisher too, and has more technique than people credit him for. Rossi would be a second for me, though i don't think he scores as much as Soldado, but he makes up for that with his build up play. Negredo is the most complete one of them imo, he's good in the air, he's a very good finisher, his movement off the ball is good as well.
IMO its like this Negredo > Rossi > Soldado