Matri's role on the field in the past two years has been much more difficult than Quag/Seba. Matri has to play with his back to the defenders and try to control the ball in the most difficult situations whereas Quag and Seba often have the ball on their feet since they're SS and have much more space to accelerate and create yet they failed big time. So if we compare Matri's role to the other two I'd easily say Matri is the one to keep since you also need Liorente's backup.
Vucinic is smarter and more talented than Quag and Seba therefore there's no question he should stay over them unless good cash is on the table.
I think with Liorente, Tevez, Vucinic, Matri things look pretty good. However, I don't think Conte will let Seba go.