what is the advantage of a 3 die set over a 2 set?

None, zero. I do not own any for rifles.

Only neck sizing brass is asking for trouble.

2 die sets are usually a FL sizer and seater die for bottle neck cartridges. Some include a neck sizer die or wit Lee dies a factory crimp die.

Straight walled cartridges will have 3 dies Sizer, case mouth flaring, and bullet seating/crimp and lee has a 4 die set with the factory crimp die included.

thank you

How so?
Genuinely interested not an attempt to argue.

I've neck sized for my rifles since, well forever, never had any problems I can recall.
Bump the shoulders and or trim when necessary.

What am I missing by not using a FL die?

You already asnwered it with "bump shoulders when necessary". If you ONLY neck sized, you will eventually wedge brass in a chamber. I have not had to do it, but I have had to tell others to do it to their rifle. Butt on the ground, muscle the bolt handle up, and stomp on the handle with your boot. The handle will break off, or the bolt will open. Terrible sight to see.

I set FL sizing die to bump the shoilders back .001" to .002" from fired length. I'm still fire formed, I hold more powder than virgin brass, the ammo easily chambers, and easily ejects, and I'm not over-working brass. Therefore my statement of NEVER neck sizing only, there is no point in it.

After many years of trying everything, I finally went over to Fireman’s approach.

Stay away from bushing dies too, you'll eventually end up with donut in the neck then brass prep requires more tools and time.

I primarily use bushing dies.

Donuts happen when you try a major neck down, like a .30 cal neck to 7mm. Then use a standard die. No wildcat, just sizing brass that is the same as the die youre using, I don't get the donut. Screw the die cap down to the bushing, then rotate back 1/8 turn and go.

I experience donuts mostly with .284win which is first necked up to .284 from .264 (6.5x284).

That is puzzling.

I find if I turn the necks it helps to prevent donuts but usually they show up after 4 or 5 loads. I had Whidden make a custom die for me and I never experienced donuts again.

Ive got some 40 rounds of 300 wby cases that have 10+ mid power loads on them not annealed and sized with a RCBS neck sizer die and a FL size every 3 or 4 loadings. Never had an issue with them.

I hope you don't.

And maybe I hope you do some day. Then you will learn the lesson the hard way, instead of the easy way. Watch someone, or you stomp on a bolt handle to open the bolt, and you'll never neck size again.

That's got to be ugly to watch, never witnessed it before. I shoot Fclass matches when there convenient (kids consume extra time now) and everyone I shoot with bumps the shoulder back .002, maybe +10yrs ago you'd find most people using neck dies but times have changed and evolved in the handloading world.

In all fairness I haven't shot the wby in probably 7 years. I haven't loaded for it in probably 10.

The ammo shoots just as good with a .001" to .002" shoulder bump, but is far more reliable chambering and ejecting.

Everything is relative and the answer "it depends" is the correct answer to most reloading questions.
I have used Lee Collet dies which just neck size on 17 Remington and .221 fireball for years and you can usually neck size 8-10 times before they get really tight to chamber. If you run max loads on bigger volume cases i could easily see issues that JG mentioned happening after a few reloads. I normally FL size as mentioned above with minimal shoulder setback and using dies that don't work the neck too much. I have bushing dies but currently don't use them as I switched to Whidden or forster FL non bushing for most target stuff.

Here's some info regarding the nk sizing vs full with a bump. This guy has a pretty well established reputation in f-class. The second video interviews many world record holders and asks what they do as far as sizing.

https://youtu.be/lLG2kSrD40g

https://youtu.be/vaqg4sJvg24