Ischii: Probably the safest of the three, because he pitches in a pitcher's park and never tends to get bombed. He also gets a nice number of K's and pitches for a decent ERA. The problem with him is he walks something like 6 batters per 9 innings, so a WHIP of 1.40 is the best you can hope for.
Graves: It's unclear how successful he'll be as a starter and he pitches in Great American, which is a scary park for a pitcher with homers left right and center. He does have nice upside.
Webb: He is still unproven IMO, and pitches in a hitters park for a team with a weak offense. Also, once Johnson and Kim are back, he will have to go to the minors (Johnson, Schilling, Kim, Dessens, Batista would be the rotation), since there'd be no spot for him.