Do you fully understand 2:1? Figuring out 3:1 shouldn’t be too difficult.
If you figure it out you’ll truly know it. If I tell you, you’ll forget it in a week.
I’ve used it on a few pitches. It’s soul crushingly slow but if you are a really tiny person and are hauling a big load it might be the only way to go.
Btw, One of the smallest people I know, and who is a multi El Cap wall vet, looked me in the eye and said “I may be small but I’m badass”. She never hauls 3:1.
The first is a 3:1 Hauling Ratchet, invented by Chongo and described in his book. Instead of a static "Zed-Cord" you need a "Double-You-Cord". You also need an extra pulley, and the end of the Double-You-Cord will be attached at the bottom to the top of the inverted ascender, instead of at the top to the anchor. Get it??
The other way is you have to turn the entire haul line into a zed, and two pulleys go right on the haul line. It is a colossal pain in the ass to set up, and then when you haul, you have to continually "reset" the inverted ascender plus pulley. It is very difficult to operate above a ledge, too.
Take if from all 125 lbs of Mark above - YOU DON'T NEED 3:1 Hauling on a big wall. Learn how to operate a 2:1, and add your partner on the free end of the haul line for added help. In hauling THOUSANDS of pitches, I remember using a 3:1 only a few times on my first dozen or two walls when I was still a n00b. If I knew then what I know how, I never would have used it. I would have done a better rig, and used the 2:1.
I think it's great you want to learn these systems. You mentioned you'd only use it to haul up heavy loads, but i'd suggest you think about other very real, very possible situations you would need to know these systems.
1. Follower gets stuck and can't do a more: A 3:1 system is very simple and easy to set up to help them thru a move and keep on trucking. 2. Follower falls on a traverse and has no holds to climb: Same idea- 3:1 or 5:1 may be needed. 3. Emergency situations: Many scenarios, but knowing pulley systems can be critical.
There are many great youtube video's on 3:1 and 5:1 systems. I suggest getting some self-rescue books, watching video's, and practicing REGULARLY! I won't get into my own epic, but in my ignorance, not knowing these simple systems, made for an ordeal.
Anyways, not trying to scare you, but just some words of wisdom i thought could be beneficial.