Misfire after replacing the spark plugs and coils?

Hello, what brand of plugs and coils did you install? And was there a reason for replacing the coils? If you put the old coils back those cylinders do you still have misfire codes?

Okay, well, you didn't use aftermarket parts so that's good. Swap out the plug and coil from #3 and see if the code follows. Do you have a scan tool that can read live engine data? The torque spec on the plugs is only 11ft lbs (132in lbs), so don't overdo it on those.

Has the p0420 been there since you bought the vehicle?

Okay, if you can, graph the rear oxygen sensor voltage on Bank 1, or both banks if it only has 1 cat. After you get your plugs and coils swapped around, see if the rear oxygen sensor voltage is fluctuating 0.2-0.8 volts roughly. It shouldn't be, it should be a steady voltage line, if its fluctuating then the cat is having trouble storing oxygen due to the misfires occurring. Hopefully that code will clear along with the cat once you get that misfire taken care of. I would think that a 2012 would also have intentional fuel injector shutdown when misfires are detected, but I'll have to look that up.
Verify the plug gap as well. If it comes out with soot or wet with fuel it might just be a bad plug. Parts are becoming less reliable these days and we see a lot of bad ones right out of the box. If the code follows the plug and/or the coil, I'd put the old one back in for now, so not catalytic converter damage occurs.

Oh okay, service info was incorrect on some of this. I would replace any intake gaskets as well if you're having to pull the upper intake off. And then retorque everything to spec. A vacuum leak on the back of the intake will cause a misfire on #3. Do you need the torque specs and sequence for the intake manifold bolts?
This is just like my truck; the number 3 is very difficult to get to.

Here's the torque sequence and amount. Definitely put a new gasket on as well.

No problem, once you get the misfire squared away, its also good to reset the Keep Alive Memory of the PCM, so it will relearn its fuel strategy. But we'll go over that once the misfire is gone.

You should be able to reset the KAM with a scan tool. It will reset the Readiness Monitors as well; I'll post the info for that here really quick. First try erasing all codes and see if the Long Term and Short-Term fuel trims start out near zero%. If there's a code still stored, a continuous DTC like the p0420, the memory won't reset until that issue is cleared up. So, if that code is sticking around. I'd try a drive to get it hot and see if it will clear up. Sometimes they will sometimes not. It depends on how hot the cat got during the cylinder misfires. But here's the Readiness Monitor info as well, they need to be complete to get through inspection.

Did you reset the keep alive memory for the PCM? And do the Readiness Monitors all read complete/ or Ok?

If you notice that the rear 02 for that bank is fluctuating its voltage and the other rear 02 is staying pretty steady, then the converter may be one its way out, but also monitor the Long Term and Short Term Fuel Trims. They will be data PIDs in the live engine data list, if Bank 1 has either a high positive or negative fuel trim (so +/-10% or more), that can cause the converter to fail prematurely, it could also have been the misfires caused by the faulty new plugs, which honestly is a real disappointment I dont think that is your fault by any means. I would even go so far as to argue your case, because you expected new parts, and bad new parts caused misfires, which if you had driven around with those misfires occurring for awhile, that will cause the cat failure. Any unburned fuel getting into the exhaust that overwhelms the cat will cause it to start to melt down due to over heating.
What I do with codes P0420 or P0430(bank 2) is visually inspect the cat with a borescope as well, or you can take the exhaust apart just to see what the front of the cat's substrate looks like. It should be a nice honey comb looking substrate. But if that Bank 1 is still rich for some reason, Fuel trims will tell you. The cats are supposed to be able to store oxygen to help them burn off the extra emissions gases. If its too rich and cant store oxygen, a P0420 will set.
It seems like the cat failed pretty quick, although I dont know how long it was from the time you changed the plugs and coils to these misfire codes setting. Technically, the PCM should have been able to compensate somewhat for the misfires by either shutting down the injectors or limiting fuel to those cylinders to prevent the cat from failing. If it was only a couple days and the codes showed up, and it really was bad parts that caused this, whoever sold you those bad parts is almost responsible for the cat failure as well, they arent cheap.

Theres 2 rear 02s, compare their voltage signals.

Soap is not going to do anything, it's a matter of the condition of the cat on that side, they are made of precious metals such as Platinum, Rhodium and others that can break down the emissions gases into CO2 and water. So, in your case where there was unburned fuel entering the exhaust due to the misfires, that excessive fuel causes the cat to overheat and melt the substrate. hindering it from storing the oxygen it needs to burn off normal emissions.
There are Catalytic converter cleaners you can try, that are specifically made for that, although I have never tried any, because by the time a p0420 keeps setting the cat is usually done by then but you can try. Just google "catalytic converter cleaner" and you will see a bunch of different ones. But if you take down the exhaust and look into the front of the converter and the substrate is looking melted, where if you put a flashlight through the rear and can't see much light coming through the front, the cat won't last much longer.

When contaminants such as coolant from a leaking head gasket get into the cat, that's silicone and could potentially be cleaned out, but give it a try, take some pictures of the condition of the cat as well, I would be interested to know if any cleaners work.
This is a good page that helps to explain the oxygen storage of the cat.
Looks like AutoZone and Advanced auto carry these type cleaners. I would try one that's not a fuel system additive, This video I searched for shows a guy pulling the front oxygen sensor and using a compressed air container, in which he adds water and carpet cleaner, I wouldn't use carpet cleaner, but you could use catalyst cleaner with a compressed air setup like he has so you're spraying directly on the cat, and the cleaner doesn't need to go through the entire fuel system first. But if the Fuel Trims are not below 10%, that needs to be dealt with first.

https://www.youtube.com/watch?reload=9&app=desktop&v=zN2POfvCBOo

https://www.vehicleservicepros.com/service-repair/the-garage/article/21172998/catalytic-converters-not-always-the-problem-with-catalyst-efficiency

What happens to the Short-Term Fuel trims if you raise the RPMs to 1,500 RPMs do the numbers come down closer to 0%? Since the Long Term is staying around zero, The Short Term is current correction, whereas Long Term is a more learned fuel strategy to deal with some issue that the PCM is constantly seeing. If the Short Term comes down to 1,500 RPMs, it's most like a vacuum leak. You may have to hold it there for a minute or so.