the only way you could use one bushing die for the 6mm, .257, and 6.5-284 is if all the barrels are head spaced exactally the same or it isn't worth readjusting the die all the time. but if they are then one die could work with a bushing and/or expander change.
@Tac-O I have seen your other post and will let you know that my Whidden 284 bushing die has a through hole without bushing of .334 if that helps you on your quest
with a little patience you can open that up a couple more thousands
Darn. My fired unsized neck diameter is 0.342.
I would think the opening for the neck would be pretty similar for most brands too