The way this spell is worded, if the creature was already restrained by another means, would they automatically be making saving throws whether they fail the initial saving throw or not?

Ex: Someone under the restrained condition from Ensnaring Strike Succeeds on their initial saving throw against the flesh to stone spell, but they are still a restrained target because of the prior condition therefore they would continue making saving throws until either the ensnaring strike ends no longer restraining them, or until they succeed or fail 3 times on flesh to stone

Quote from TrevMan101 >>

The way this spell is worded, if the creature was already restrained by another means, would they automatically be making saving throws whether they fail the initial saving throw or not?

Ex: Someone under the restrained condition from Ensnaring Strike Succeeds on their initial saving throw against the flesh to stone spell, but they are still a restrained target because of the prior condition therefore they would continue making saving throws until either the ensnaring strike ends no longer restraining them, or until they succeed or fail 3 times on flesh to stone

That's an interesting point. I would say yes, that works. Because it doesn't say "A creature restrained by this spell", it says "A restrained creature". And it's worth noting that it also does not say that the spell ends if they succeed on the first save.

However, again based on this specific wording, I'd say that even in the case where they're already restrained, they still need to roll that first saving throw, because it seems like it counts as a success or failure toward the total.

Does a creature Petrified by this spell still count as a creature or is it now an object? I have players wondering if they could use the Gust cantrip on a creature that was Petrified to move it. 

It is still a creature, it just has the petrified condition.

Does anyone know which source book grants this spell to Sorcerers and Druids.  It's listed on their pages as additional options but I'm not seeing it in Tasha's or Xanathar's.

Quote from PollyNim >>

Does anyone know which source book grants this spell to Sorcerers and Druids.  It's listed on their pages as additional options but I'm not seeing it in Tasha's or Xanathar's.

This spell is on both the Sorcerer and Druid spell lists in the Player's Handbook. 

It was not on either of those lists in the 2014 rules, if that's what you're getting at.

There's a section on the 2014 Sorcerer page for Optional Class features that lists additional spells that weren't included in the PHB's sorcerer spell list.  It mentions Tasha's and Xanathar's as a source for these inclusions, but neither of those books mention Flesh to Stone.  It can't be from the 2024 version of the PHB either since that would be with the 2024 Sorcerer instead.

Quote from PollyNim >>

There's a section on the 2014 Sorcerer page for Optional Class features that lists additional spells that weren't included in the PHB's sorcerer spell list.  It mentions Tasha's and Xanathar's as a source for these inclusions, but neither of those books mention Flesh to Stone.  It can't be from the 2024 version of the PHB either since that would be with the 2024 Sorcerer instead.

Right. Those two books added various spells to various class lists via optional features. Neither of them added this specific spell, Flesh to Stone, to either the Sorcerer or Druid lists. The book that did that was the 2024 Player's Handbook.