Has anyone gone from size 3X to S?

I would imagine the ability to fit in to a small would be more of a skeletal size thing.. if you have a small skeleton then it very well may be possible! Personally as a large framed person, I don't think I could ever fit in to anything small. Medium maybe.
I could be wrong though but that's just my thought. No harm in trying so long as you don't throw yourself underweight and unhealthy just for a dress.

Most people look better with enough skin on their bones so that their skeleton features do not show (such as having boney shoulders or hips). If I like a dress or other outfit and it comes in several sizes; I will try on what I 'think' is going to fit me (until I find my size in it) and if I still like what I look like in the mirror, then I will buy it. You can't tell by size label alone. Depending on what manufacturer makes it will have affect on label size. If you have an outfit that you like and you don't like the size of the label (or maybe embarrassed by it) then cut it out. Find outfits that flatter you, no matter what size you are, . . . if you are a heavy person shop in a 'big woman's store'. Better to have an outfit that is stylish and 'in your size' than trying to stuff yourself into a size that is really not cut to fit you. I go to stores, sometimes, with my D-n-L and see the frustration of trying to find something for herself - she is a 'big woman' - large breasts. While I have suggested going to a 'big/larger woman's shop she does not want to do so. I've looked at clothes in them and they are very fashionable and they are priced along the same lines as regular sized clothes in most department stores. Whether you are fat or skinny depends a lot on how your clothes fit you (pattern, color, and cut). Finally, they are showing models who are larger than Twiggy. Nobody is ever going to look like any "Barbie" . . .

I would think at 5'9 it would be hard to fit into small. I am 5'6 and at my most slender (people told me I was too thin though I don't think I was) I wore a medium. Not just for "filling out" but think about sleeve, pants, and skirt length too.

Why would you want to set a clothing size as a goal, especially when in the US there is absolutely no consistency in sizing to begin with?

Healthy weight, BMI, measurements, how you look/feel, would be more ideal, IMHO.

NEVER say NEVER, it's far reaching but if you want something bad enough it will happen ! !

Using clothing sizes is pointless, regardless of frame size skeleton. Clothing manufacturers are doing lots of vanity sizing lately. Even within the same company like H&M, go to their «youngster» dep and the size M is considerably smaller than size M in women's dep. Or even more frustratingly, try on two different dresses in both size S from same dep and they still fit differently. It's all bout the cut/design.

It's kinda annoying, cause if clothing companies could agree on a standard for rmeasurements...it would be easier ordering online. The closer the fit, the harder to get right. Think swimwear or underwear.

I'm 5'7" and even at 125 pounds (many years ago) I wore a medium shirt or dress. Shoulder width. Chest size. Some things just don't really reduce. Not that that's always a bad thing.

I went from a 3x to a medium in 10 months so I'm assuming I'll get to a small. I can fit into small shirts at victoria secrets, but I think they run big

I went from a 24 (2x-3x) to a 4-8 (S-M)

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As a 5'7 male, I went from borderline 2xl (shirts) and I am at medium right now. That's with about 20 or so lbs left and can already tell I will be a small because a lot of the medium clothes is starting to feel roomy!
Losing weight = big money on clothes lol

All depends on frame size. I'm 5'9 and have a large frame. I don't recall ever being below a size M even at my lowest weight. My friend however is almost 5'11 and has a super tiny frame, before having her daughter she would fit in XS tops.