Tom,
Get ready for a quick 3-gun history lesson.
Initially, there were only two divisions. Open and Limited. This would be mid to late 90's. Open-anything goes. Limited-iron sights only, no comps. With the 94 AWB in full effect, manufacturers started making AR's with muzzle brakes and comps, perm attached. This caused a problem because every new shooter to the sport would automatically be put into the open division because of a comp they couldn't take off the rifle. Allowing them into Limited keep new blood coming into the sport, but gave the new rifles a significant advantage over the standard pre-ban flash suppressor rifles, so they changed the rules to allow comps in the Limited division. At the time the "best" comp on the market was the JP tank brake comp. It was and still is very good. Better than the factory comps available at the time. Since shooters with post ban rifles couldn't change out the comps on the rifles, shooters with pre-ban rifles could use the JP comp while post-ban rifle shooters could not. There was enough of a design advantage that having a pre-ban rifle with a JP comp was still enough of an advantage over the factory post-ban rifle/comp models that it was still deemed unfair to all those who couldn't acquire a pre-ban, but the powers that be didn't want to outlaw the JP comp across the board. In the end, they created the 3" rule to prohibit the JP comp from being used in Limited division, but it would still be legal for Open division.
This rule carries through to today, but now there are several new comp designs on the market that are just as good as the JP comp, but fit within the 3"x1" rules, so its more of a carry over rule today, but at the time it was a significant issue.
USSA-1