Three jackets and I am soaked inside every one of them: is staying dry in sustained rain actually possible?

There is no fourth jacket that solves this, and understanding why will save you the money.

How a membrane works. It passes water vapour outwards driven by a difference in humidity and temperature between inside and outside. That is the whole mechanism, and it has a hard ceiling.

Why it stops in sustained rain. Two things happen and they compound:

The face fabric absorbs water once the durable water repellent finish stops working. A wet outer face means the outside is now saturated, so the humidity difference across the membrane collapses and vapour transfer largely stops. That is wetting out, and it is what you are seeing at the one hour mark.

At the same time you are walking uphill with a pack, producing sweat far faster than any membrane can move it even in perfect conditions. The rates are not close, a person working hard produces more moisture per hour than the best membrane can pass.

So in sustained rain, going uphill, you will be wet. The honest thing a rain jacket does is keep the wind off and stop you being cold and wet, which is a genuinely different and much more important thing.

The expensive jackets are not a con, they just solve a problem you do not have - they are better in cold and light rain, and no better at all in your scenario.

The thing that changed sustained rain for me and gets laughed at every time: a trekking umbrella. In steady rain without much wind it keeps your head and torso dry while your jacket stays ventilated, which is the combination no membrane can give you.

Useless above treeline in wind. On a forested trail in three days of drizzle it is the single best piece of kit I own.

Three jackets and each one more expensive. That escalation is the tell that the category is the problem.

Once you accept the above, the goal changes from staying dry to staying warm and controlling how wet, and everything gets easier.

What actually works, in order of how much difference it made for me:

Ventilate aggressively. Pit zips fully open, front unzipped to the sternum, cuffs loose, hood back when you can. Most people wear a jacket sealed up and then blame the membrane. Moving air removes more moisture than any fabric technology.

Wear less under it. Being slightly cool at the start of a climb is correct. If you are comfortable at the bottom you will be soaked at the top.

Consider a poncho or a very loose shell for warm rain. Enormous ventilation, covers the pack, and the airflow does what a membrane cannot. Poor in wind, which is its real limit.

Give up on being dry from the waist down. Rain trousers in sustained rain are the same problem worse. Many people walk in shorts and accept wet legs, because legs generate heat and dry fast.

Keep a windproof layer for stops, since the moment you stop moving is when wet becomes dangerous rather than annoying.

The mental shift is the main thing. Wet while moving is fine. Wet while stopped is the problem, and that is what you plan against.

The rule that turns a wet three days from miserable into merely damp: one set of clothes never gets wet, ever, under any circumstances.

Base layer, socks, and something warm, in a dry bag, and they do not come out until you are in the tent with the door shut. Not for a cold ten minutes at lunch. Not because you are shivering at a break. In the tent, or not at all.

The discipline is the hard part and it is the entire trick. People break it on day one at a cold lunch stop, and then have no dry clothes for three nights, which is where a wet trip becomes a bad one.

What goes with it:

Put the wet clothes back on in the morning. Genuinely horrible for ninety seconds and then irrelevant, because you warm up within ten minutes of walking. Everyone hates this and everyone does it.

Dry bag the sleeping bag properly, and treat that as non-negotiable in the same way.

On maintenance, since your jackets keep wetting out: wash and reproof them. The repellent finish degrades with dirt and body oils, and a jacket that beads for one hour is one that needs cleaning, not replacing. A wash with the right detergent and a reproofing treatment restores most of it and costs a fraction of a fourth jacket. Do that before you spend anything.