I think all three over the next year and a half.
Baring a illness or death, I think the first to go will be Associate Justice Ginsberg. Followed by Associate Justice Kennedy, then Associate Justice Breyer.
Ginsberg has been threatening to leave for several years now. Kennedy has also been making some "exit stage left" noise.
Now with that written: Who is it to say that a current Constitutionalist would not take the opportunity to leave with Mr. Trump in the White House.
Now my opinion...I do not care if a nominated Associate Justice is liberal/conservative or Democrat/Republican as long as they are a Constitutionalist and did not feel that they had a legal right to legislate.