Flesh to Stone states:
You attempt to turn one creature that you can see within range into stone. If the target's body is made of flesh, the creature must make a Constitution saving throw. On a failed save, it is restrained as its flesh begins to harden. On a successful save, the creature isn't affected.
A creature restrained by this spell must make another Constitution saving throw at the end of each of its turns. If it successfully saves against this spell three times, the spell ends. If it fails its saves three times, it is turned to stone and subjected to the petrified condition for the duration.
My reading of this is that the initial save against the spell counts as one of the failed saves so if the target failed an additional two saves it would be petrified. Is this correct or is the correct interpretation that the target must fail three of the subsequent saves to be petrified?
the spell Contagion uses a similar mechanic but has slightly different wording that makes it clear the first save is not counted.
At the end of each of the poisoned target's turns, the target must make a Constitution saving throw. If the target succeeds on three of these saves, it is no longer poisoned, and the spell ends. If the target fails three of these saves, the target is no longer poisoned...