A triple locking carabiner (also known as a “twist lock”, or “triple action” or a “tri-act”) can be a good choice for certain climbing uses. And, not such a good choice for others . . .
The idea behind a triple locking carabiner is that there are three separate actions that are required for the gate to be opened. Typically, that’s sliding a metal sleeve, twisting the metal sleeve, and then finally opening the gate.
The Petzl William is a triple locking belay style pear-shaped carabiner.
The Petzl Am’D is a more all round D shaped carabiner.
Twist lock carabiners have some nice features . . .
1 - When the gate is closed, you know that the carabiner is locked, and that’s it’s very likely going to stay that way. With a normal screw gate locker, there is always that little nagging background voice that asks “Well, the gate is closed, but is actually locked? And, if the gate rubs against the rock or some other weirdness, is there a chance it could unlock itself accidentally?” If you use a twist lock carabiner, those two concerns are much gone. This makes them a good choice for beginner climbers, because you can be pretty much sure it's going to work as intended, without reminding the beginner all day long to be sure and lock the carabiner. That's one of the reasons guides like to use them for clients.
2 - It’s easy to open the gate even after the carabiner has been severely loaded. If you put a big load on a standard screw lock carabiner, tighten the gate, and then unload the carabiner, sometimes the screw sleeve locks in place and it can be tricky to open. No such issue with a twist lock.
Use of triple action carabiners is common in rock gyms, climbing schools, industrial rigging, high rise window washers, mountain rescue, arborist work, and with People Who Do Dangerous Things Far Off the Ground, so that ought to tell you something about how safe they are.
But, safety does not always mean ease of use. Let's read on.
They also have some not-so-nice features . . .
1 - They are not well-suited for snow/ice climbing, because A) they can be difficult to open if you're wearing gloves, and B) bits of snow and ice can get stuck in them, causing them to fail in various ways. (Yes, people have been known to pee on their carabiners to get them open, but I don't have a video tutorial on that one yet. =^) The same concern applies to dirt and mud, so probably not so great for caving and canyoneering, but I don't have any direct experience with this.
2 - They are pretty darn hard to open with one hand. Often it's helpful, especially at a more complicated rock belay, to be able to operate the carabiner easily with one hand, and dealing with too many twist lock carabiners can be irritating.
3 - They are a bit heavier and more expensive than a standard screw gate. Below: Petzl Am’D 79 grams, Black Diamond screw gate, 55 grams, difference of 24 grams. Probably not a dealbreaker for most people, but if you're a lightweight nerd, it might be a consideration.
So, what's the best use for twist lock carabiners?
Should you use them for every part of rock climbing that requires a locker? No.
Like almost all aspects of climbing, it comes down to personal preference, so here's mine. I have several twist lock carabiners that I use for:
Setting up top ropes in an instructional setting where I'm not going to be around to keep an eye on them
For my personal leash/lanyard when rappelling
For my primary belay carabiner
Various things on big walls, where large loads hanging at strange angles and super-secure connections become more common and important (such as fixing the rope for the second to ascend, or connecting haul bags to each other and the haul rope.)
Here's the easy solution: buy a few and see how they works for you.
One other option - try the more modern flavors of auto locking carabiners. I'm not going to try to summarize them here, but many of these have some innovative designs to try to address some of the problems with twist locks mentioned above, such as not working well in the snow, or being able to open them with one hand easily. I have both of the carabiners below and they work great.
If you want to read more about this, here’s a good summary of the different flavors of auto locking carabiners.
Black Diamond Magnetron. Very clever design uses magnets to keep the gate closed. Easy one hand opening, works with snow and ice.
Edelrid Slider HMS carabiner. A great belay carabiner, easy one handed opening, supposedly works in snow and ice (I have not tested it yet.)