I'm thinking of giving three under a try. Are any of you able to tune your bow for both styles or does it change your tune a great deal?

Generally you should be able to tune your bow for either style, but I'm not sure you could tune it ideally for both at the same time.  3 under generally requires a higher nocking point.  You might try a 5/8" nocking point, which might be a little high for split and a little low for 3 under, but okay for both.

Mostly just a higher nock point from my experience.  Bow is quieter for me with split but I just shoot much better 3 under.  Not to add confusion but I hear a lot of good about "2 under" lately.....

Thanks Dave. Shed, I'm not sure I could shoot with only two fingers.

Not in my personal limited experience and expectations.

Not the same thing as you asked but I have a friend who shoots three under.  He offered to sell a really nice Tall Tines bow to me.  It is tillered  for three under. I shoot split finger.

I tried shooting it with the nock points set up where he had left them.  It shot just fine. Good flight, I felt good enough about it that I used it this fall. Maybe it fell into the parameters set by Mcdave, never measured, it just worked as is.

Curios now, I should measure it for future string set ups.

For a while in the '80's I was drawing split and then straightening the index finger out after anchoring.  It worked great with one particular bow of a light weight but strength was a problem with the heavier bows.  Ended up back to typical split finger.