I can't think of an easy way to tell which is stiffer. Springs are rated on spring rate, so you could place the same weight on top of each spring and measure the difference in compression. However a 30 plus year old spring that has been tensioned likely won't spring back to the same free length as the springs you just got in the new 3 shoe.
Like the wiki says 2 full turns out from full compression is the tightest you can go and expect the clutch to function. I have run one set up like this on my 65 metra and it runs fine, revs up a bit but is a good compromise.
You can get different springs that are stiffer, I can not speak to there function.
The main thing is make sure all 3 have the same tension, so all 3 pads engage at the same time.