Lifetime ERA/FIP/xFIP
Schilling - 3.45 / 3.23 / 3.17 (Postseason 11-2, 2.23 ERA, World Series 4-1, 2.06)
Mussina - 3.68 / 3.57 / 3.68 (Postseason 7-8, 3.42 ERA, World Series 1-1, 3.00)
Pettitte - 3.85 / 3.74 / 3.70 (Postseason 19-11, 3.81 ERA, World Series 5-4, 4.06)
Of the three, Schilling is the best.
I was lukewarm on Mussina, but after looking over his stats again, I don't think his inclusion is egregious.
270 wins, 2800 K's, 9 top-6 Cy Young finishes, 83 bWAR including 10 5-WAR seasons. The only knock on Mussina is that he never won a Cy Young or a World Series.
Pettitte is a no. His counting stats are close to Mussina, but only 5 top-6 Cy finishes, 61 bWAR, and only 3(!) seasons with >4 WAR. He was solid, but rarely outstanding. The biggest thing going for him is being the 5th or 6th best player on a team that won 5 World Series. That's not enough for me.
Schilling is tough, but I ultimately lean towards yes. Paltry win total (216), but he got 3K strikeouts. Only 4 top-4 Cy finishes, though tough luck in two of those years, 2001 and 2002, when he had over 17 bWAR total and probably would have won it if Randy Johnson was in the AL. 81 total bWAR and a clearly defined peak , unlike Pettitte, gives a HOF feel to his career. Add in that the postseason heroics and I think he belongs. He was one of the defining pitchers of his era.