A couple thoughts. Archetypes were a great idea in Pathfinder, yet easy pickings to cherry pick features when multiclassing. For example a trickster / caster rogue would pick the lightly armored fighter archetype and then have canny defense for the win. Is it overpowered to allow dex and int to AC? Maybe not in the average case, but for someone who might have rolled two 18s or two 17s at level 1, a human variant would have 20 AC for the whopping cost of 5gp. A Barbarian could do that, but he needs strength boosted too, so he has a three stat dependency. Monks need dex and wis only, so this is roughly equivalent to what a roguish combat wizard would have, with a 1 level dip. What's better than concentration saves? Not getting hit in the first place. This is going to be a power pick for wizards or other casters, for sure. And you don't even have to take the fighter level dip at level 1 to get all the armor prof, which makes Uncanny Defense more applicable to more characters at more levels and therefore much more powerful. A wizard would have 20-22 AC within no time.
The second issue I have with it is the idea of archetypes taken at level 3. That's a semi-good idea, given that's what the subclasses do already. A better subclass way to regain the use of lost or unused armor proficiencies from the base fighter would be to simply supercede them. Picking up one level of fighter for uncanny defense is too good I think, it would be better to have this at level 3 in my opinion, even if that goes in a different direction than previously.
The only thing that really keeps this stuff from completely obsoleting the base fighter is the great weapon master feat. As soon as you realize that archery style plus kensai plus sharpshooter gives three attacks at level 5 with superlative bonuses to hit and damage (when dual throwing daggers), it's going to start making rogues jealous how much damage you can do without even having allies near the opponent. I would even multiclass with swashbuckler rogue to never get another OA while bladesinging.
With action surge you could probably attack 10 times in the first round of combat with no chance of retaliation. That's if you swashbuckle but if you don't you still need to ask yourself, is a feat a good thing to give that is nearly as good as a subclass feature?
Dex is already king in D&D, except for Great Weapon Master. Perhaps great weapon master being so powerful establishes many other seemingly powerful options.
A much simpler approach would be to remove medium and heavy armor proficiencies and give them + 1 to AC. That would still be a good thing for rogues to pick up, but not a must have multiclass selection for wizards and basically any and every non heavy armor wearing class in the game. A single level dip for up to +5 to AC (adding Int to AC in an urban setting where armor is banned)? Sure, I'll take that. As would everyone. A good sign it would be overpowering.
Getting better AC than plate, no speed or disadvantage to stealth checks or anything. This is a strict power boost to casters and rogues. Heavy knights have no purpose. It's fine for an urban game being that heavy armor is something only palace guards might wear in most cities, but then even leaving down to go into battle in a war scenario would be strictly superior to have an army of uncanny defense swashbucklers.
Sorry for the rant, this is way broken.