I Have No Clue
Here is where I would ideally stick a free agent starter, especially if it’s someone on a short term contact. One of my favorite targets for this spot is Lance Lynn from the Cardinals, or Andrew Cashner.
Both are proven pitchers, who just put together strong seasons, with ERA relatively under four. The Giants really need some, not necessarily Ace-Like production out of the third spot, but someone who can go out and get quality starts, consistently, and keep them in ballgames.
If they have to sign a free agent because Johnny Cueto decides to opt-out, then I actually think Jeff Samardzija stays in the third spot. Matt Moore has clearly lost that place in the rotation after his dismal 2017.
It was a real regression for him, and he may be sent out in a trade if some team is willing to take him on as a project. Ultimately, a Cashner or Lynn type signing would be the best way to deepen the rotation down to the third spot, as it would/should push Samardzija down to fourth.