Strange. I just see the three. Trump’s not there. Seen screenshots from other people with the same result. I tested it prior to posting.
Trump left out of Google search for presidential candidates, along with a Snopes article on the same subject, speculating that the result might be because Trump is no longer a candidate, but a nominee. And I still saw the same four candidates, as well as collages of all the candidates this round.
Maybe the difference is that when Google auto-filled, I actually searched "presidential candidates 2016"?
I don't know, but it is strange. Google results do change, depending on how you search.