What Pine says is true, but you're really not moving your draw hand that much in switching to three under. Given that one tends to raise the nock point for 3-under, the ring finger is maybe a half-inch lower on the string. Yes, it will increase lower limb bend a tad, but usually not enough to visually notice. Sounds like some other change in your form also might have occurred. The way you grip the bow or a change in where pressure is exerted on the grip, a change in anchor point, perhaps. If you're shooting very short bows with a long draw length, I suppose it might be noticeable, but folks have been making that switch for ages, and I've yet to read where they've noticed a substantial change in limb bend. Enough to affect limb timing and noise, yes, but usually not enough to see ("a ton of extra limb bend").
BTW, how do you manage to see the bend in the lower limb when you're shooting the bow? Pretty hard to do. Maybe the changes you make to shoot 3-under just give the impression that limb bend has changed substantially. Maybe you can see it better/from a slightly different perspective shooting one way vs. the other. I dunno. It's got me stumped.