I picked up 3.1 Shoulder, Elbows, and Shinguards around american thanksgiving. I was on the IR/COVID list for a bit, so I've only played two games with them. I'm 6'2", 185-190lbs, kinda thin build. I tend to be a medium in volume for most stuff, and a large in length, and often go the medium route. For all three sets of pads, once I put them on, I didn't think about them being new and didn't notice them while playing at all. My previous elbows and shoulder pads we probably 12ish years old. I've played with other stuff, and nothing felt as good as those. These, I like. Other new pads I've worn I keep readjusting during the game, until they eventually conform to my body. All of these felt like they fit my body right away, and I didn't find myself readjusting anything once I got on the ice.
One thing I noticed when I first tried them on, it felt like all of the strapping was appropriate size for a medium. Because of my body size/shape, in other brands I often have shin and elbow straps as tight as they go, and I have trouble imagining what type of body would need the straps at maximum extension. It's like manufacturers create too much room to open up, but don't allow well for pads to tighten smaller. I don't have any of the straps on the tightest setting. Both the velcro attachment points and strap size means that if the straps start to stretch after a few years, I still have to room to play and retain the same tightness. That's not to say they have a shallow fit or super tight fit, I think STX was just more mindful of ways to design the velcro to offer more options on the smaller side.
Shoulders(medium):
(Previous pads Mission fuel 90, M) These fit close, but not tight or awkward. For me, the fit was comfortable, but not ground breaking. Visually, I don't feel like they are anymore low-profile than my 12 year old Mission Fuel shoulders, but those seemed relatively low profile, to me. On a profile scale, the STX are much closer to 5030s than Douglass Defenders. Bicep straps are fixed on the pads, so there's no velcro. I've always set those straps then never undone them, so I like this. I just now noticed that the bicep guards are attached to the shoulder caps with velcro, so you can move those up and down as you want. There's a horizontal strap sewn into the should cap, then another thick nylon strap loops through that, and velcros to itself. This gives a little bit of play, which I think will keep the bicep pad centered on your arm, and offer more freedom of movement without sacrificing protection. Front has a hard plastic piece down the sternum, and firm foam to the sides of that. Back is firm foam down the spine, and softer foam to the sides. On the front and back, you essentially have three areas- rigid middle piece, foam to the side of that, and kidney/rib padding toward the bottom. They are attached to each other via well sewn neoprene, which I think offers a more flexible fit, rather than all being one or two pieces. It keeps all the pads close together, but gives enough flexibility that it doesn't feel restrictive. I do kinda worry about how that neoprene will hold up over years and years. Regardless of my mirror-test rambling above, these feel more low-profile than other pads, and fit snug to your body and move with it.
Elbows (medium):
(Previous pads reebok 10k, M) The first thing you notice is the neoprene sleeve that your arm fits into, which effectively replaces a third strap. I was nervous about this, but after putting it on, didn't really think much about it. It is a full sleeve that your arm goes through. Before seeing them, I thought maybe it was just attached to the top, and your elbow rested on the pad the way traditional elbows do. The area under/around the sleeve does appear to be finished like a normal pad, so if you somehow ripped out the sleeve, it's not like your arm would be sitting against uncomfortable material.
Fit was comfortable. The forearm strap runs 360 degrees around the whole forearm and secured with velcro, which I like. Felt like the strap really secured it evenly, as opposed to most pads which just pull the top pieces together. I think this strap will accommodate more arm types. Pads are hard plastic along the bottom and back, and firm foam towards the sides and top of the arm. I worry about the long term durability of the sleeve, but looking at the inside of the pad, you might be able to rip all of it and have the pad still function. Bicep strap and forearm strap seem secure enough that they can do all the heavy lifting in that scenario.
Shins:
(Previous Pads: CCM QLT 290, 16") I bought 16" because I have longish legs and wear the pads over my tongues. They felt like they might be a touch longer than other 16". Padding around the edges and back feels more sturdy that on the QLTs I'm moving out of. Strapping on the calf is thick, and actually tightens enough around my calf. Calf strap is also wide enough that it won't create some weird obvious pressure on your leg. The strap that sits just below the knee is great. Similar to the elbows, the strap goes all the way around the shinguard, the same way tape would, so it provides a nice snug fit. You could also easily cut this strap off if you are one of those people. I do wish this strap attached a touch lower, the very top of the strap rests sort of on the back of my knee joint. But, everyone has different bodies and if it fit me perfectly, it might be too low on someone else. Flexibility and comfort are good. I do notice the top flap on my left pad catching on my pants. These are pretty thick foam, thicker than I've ever seen there. This is good for extra protection, but means I can't quickly bend/deform it to keep from snagging my pants. I assume it will break in. (I also suspect better skating form and sitting a little lower would help here. But that's another thread.) Padding in the knee seems good. I haven't taken a fall straight on them yet. There's both adequate padding in the shinguard, and the liner also has some padding in it that feels thicker than other pads.