What are the advantages of different fletching lengths? What do you guys use, 3" ,
4" , or 5" ?

I' m a middle of the road guy. 4 inchers in both feathers and vanes. Have used them for 25 years.

I also use the 4" but i use the vanes because of the fact that i dont have to worry about getting them wet.

Depends on several things.

-Type and weight of broadheads (Some require mor fletch to stabilize, and faster).
-Weight of loaded shaft.
-How close the shaft-spine is to being correct for your draw-weight.
-Finger or mechanical release.
-Bow draw-weight.
-Etc....

Some shooters can get good flight with 4, some need 5. Just about have to test-shoot your setup to know for sure. If you like the 4" vane but vanes do not do the job, try a 4" feather. If neither do the job, then you apparently will need 5" .

The correct vane or fletch size/type/angle set is critical. They are either right for your setup. or they are not.

4" vanes, tried feathers going back to vanes.

Big ol' 5" helical feathers. Shield cut. They work great for any kind of head I put on my arrows. I waterproof them if it looks like it' s going to be a damp day. If it looks like it' s going to be a monsoon, I don' t go hunting. The deer wouldn' t be moving anyway.

I shoot the devil out of my feather fletched arrows to soften the feathers up, well before hunting season. That keeps them from sounding like you' re pulling velcro apart when you brush against something with them.

I concur with Arthur P on the " Big ol 5" feathers." I have not shot " Shield" since my recurve days, but 5" feathers, IMO, are better and faster stabilizers, and are not as senstive to clearance contact.

In 39 years, I have never had the weather related problems with feathers that so many say they have.