For me, 3 Gun has been a mixed blessing as I run it with the guns I actually use (or am planning to use) for HD. So I am not the most competitive even in Limited or Tactical classes. But I have learned a lot, including keeping track of the rounds remaining, how to float a round on the carrier, and, especially, that Benelli thumb is real with my stock M1 Super 90. Rather than open up the loading port, I have learned/adopted two techniques:
1) Use the knuckle of my thumb to drive the shells into the magazine tube. This works remarkably well with my short fingers and gloves as the glove material is taut over the thumb and cannot get caught in the magazine tube.
2) Insure my thumb is straight if I want to use the thumb tip to load. Still working on this as I tend to bend the thumb under the stress of shooting on the clock.
I believe 3 Gun can be a benefit to the shooter focusing on defensive use of the shotgun IF the shooter resists the temptation to become "gamey". It is a hard thing to resist when one is getting thrashed by the competition.