Disclaimer: I do not believe the Giants will/should rely on an in-house option to play third base in 2015. Though, here are the top choices if the post-Pablo "Plan B" doesn't pan out. Also, this list excludes players who did not play in the minors last season. (i.e. Joaquin Arias).
Duvall was the shining star of San Francisco's minor league system in 2014. He slugged nearly .600 in 91 games and would have likely eclipsed the 40 home run mark if he had played a full season with the Fresno Grizzlies.
We did not see that production while he wore a Giants uni, though he never played consistent time. And despite playing more third base than first base in the minors, Duvall was used almost exclusively at first with the big-league club. Now, that could have been because filling a void at first was a more pressing need at the time. But I would guess the Giants feel Duvall is best suited at first rather than third. Still, you cannot ignore that 82 percent of Duvall's games have been spent at the hot corner since he was drafted in 2010.
Duvall has a career 91.7 fielding percentage at third, including a 91.5 mark in 2014.
Like Duvall, Dominguez came out of the University of Loiusville and has played third base more than any other position in the minors for the Giants. Though in 2014, Dominguez started just 32 of 131 games at third, which is down 48 percent from 2013. This is likely due to some experimentation with Dominguez in the outfield, in which 58 percent of his total starts in the outfield came in 2014 with the Fresno Grizzlies.
Dominguez is a little better defensively than Duvall at third, with a career mark of 92.6, which includes an improved 93.9 in 2014. But what you gain defensively, you likely lose on the other side. Though Dominguez's home run and RBI totals clmbed in 2014, his batting average, on-base percentage, and slugging percentage all fell compared to 2013. He also strikes out five percent more often than Duvall.
A 2014 Arizona Fall League participant, Miller has been a surprise late-round draft pick. Out of Western Oregon University in the 25rd round of 2013's draft, Miller reached the Double-A level this August and put up a .304/.373/.402 line in 102 plate appearances. The batting average and on-base are both above his career averages while the slugging is understandably below.
Miller has only played 28 of 162 games (17 percent) at third base, but saw some looks at third in the Fall League, where he looked composed and comfortable in the multiple games I watched. He made four errors in 16 games in Arizona (though I don't know specifics as to at which positions he made those four errors) and has a career fielding percentage of 97.7 including 95.8 at third base.
Miller is important because he's incredibly versatile. His athleticism and skills allow him to play all four infield positions. But he is, right now, a candidate to make the big-league club in September of 2015 or the beginning of 2016.
Duffy has played the least amount of third base out of the four players mentioned here. He has seen less than two percent of his 241 career minor league games from third and was positioned there just once after being called up to the Giants. Having played an overwhelming majority of games at shortstop (over 96 percent), one would think a transition to third base would be smooth. Like Miller, Duffy has good fielding percentage numbers (95.9) and is versatile, though I don't ever see Duffy playing any first base like Miller has.
VERDICT: The pros and cons are clear here. The power potential in the bats of Duvall and Dominguez are lacking with Miller and Duffy, and on the other side, Duvall and Dominguez could be seen as defensive liabilities, or at least less skilled than Miller and Duffy. In the same regard, the Louisville boys have more experience at third base.
If, and only if, the Giants need an in-house option at third base at some point in 2015, I'd say it'll either be Duvall or Duffy. It will all come down to what the team needs most: offense or defense. And knowing the Giants style of play, I would say defense will more often than not prevail.