I switched to 3-under about two weeks ago.  My bow is tillered even.  All my arrows, which were just fine while shooting split, are now distinctly under-spined. My release is clean and I am not off-center with the string.  Is this a common occurrence?  What is the cause of the change?  I would appreciate any insight.

Mark

No, move the nocking point up, 1/16" at a time. Continue to shoot 3 arrows at 5 to 10 yards into a target without grain. Shoot level if possible, rather than one below your waist.

If the nock is low, the arrow will be nock low, move the nock up in the direction you want the nock to go.

When the nock is in line with the arrow at full draw, then work on left or right nocks. Following the same procedure for left or right nocks, move the rest out if your nocks are right, move the rest by adding or subtracting thickness if you have a fixed rest.

I would go with the nock point adjustments to see what happens unless you have done that, and the arrows are still weak.   It could be that you are catching the side of your rest due to too low a nock point and the rest is kicking the back of the arrow out making it look weak.

I have the opposite.  I need a little weaker arrow 3 under.  This is probably due to a slightly shorter draw length and maybe a little more drag getting of the string split causing it to pluck just a bit more.  I for sure get of the string cleaner 3 under.

When I switched to 3 under I kept the same nock point, but could not get tuned until I added a second nock point underneath. Bare shafts hit low / flew nock high no matter where I set nocking point.

Don't recall ever experiencing a change in shaft stiffness due switching from split to three under.

Proper nock set and addition of a 2nd nock located underneath w/~1/16" clearance should resolve your issue for 3 under.

Mike's statement was what was going on for me.

My nock postion has been adjusted upward for great flight in the vertical, and I am using double nocks.

I did some experimentation this morning, and my draw is a slight bit longer when I pull three under.  My draw allows my elbow to be a bit higher, which allows my particular anatomical structure to pull a bit more with  back tension.  I used the cloths pin method and was consistently and exactly 1/2" longer on my draw.  My carbon arrows responded by a reduction in point weight of 30 gr. to stiffen them up the proper amount.

Mark

Alright Mike!

if my arrows change after I make a change, I move my silencers around instead of changing point weight.

I use wool puffs and moving them just a little will often fix the weak/stiff problem. This way I can use the heads I like and not have to have broadheads in different weights.