If I had to make a choice of one of the three, it would be Hermione.
Would be like Harry to force her to sacrifice her life to allow him to get to Voldemort.
Anyway, can't be all three so that option's out, Snape is the bad guy/good guy so he won't die and Ron doesn't strike me as the character whose death would make a plot worthy one like Hermione's might be.
