I am reloading a 30-30, 30 Herrett, and 308 now. Started out just loading the 375jdj because I needed ammo and lost control. But anyway. Will one .308 necksizer neck size the 30-30, 30 Herrett, and 308win or do I need to buy one for each different case.

As long as there is clearance for the case bodies and its not too long it will. What kind of gun? If its a t/c I know at least one specialist who suggests bumping them back for proper head space. On break open guns the frames stretch slightly during high pressure which makes the case a little long after it springs back. http://www.bellmtcs.com has a section on t/c headspacing that is very informative.... That being said i always just neck size anyways and get fair accuracy as long as its not so tight it effects complete lock up for all the internals you should be good to go.

I would recommend getting a die set for each one. On a Contender you will probably have trouble shutting it after a couple "neck size only" reloads. I know, Not What You Wanted to Hear.

Space out buying the die sets, it will not hurt so bad.
I have more die sets than most gun stores I shop at. Can't have to many.

Bob