I'll add a several thoughts to the discussion. Water, oxidation, and expiration dates seem likely the most likely issues to look into further. All good points being made here. Oxidation, water, and technique are likely your biggest issues.
My first question is do you have equipment other than what came with the Mr. Beer kit? I played with a Mr. Beer kit many (many) moons ago, long before I actually got into home brewing. It is physically possible to make decent beer with one, but you are hamstringing yourself with the equipment if that's what you're using. If memory serves, the Mr. Beer setup doesn't even have an airlock and just relies on a poorly sealing lid to let the CO2 from fermentation escape. You mentioned 3 weeks in one of those before bottling. You can do that in a decent normal setup, but in a Mr. Beer, I suspect you're looking at significant oxidation due to air coming in that poor seal. Think about wet cardboard. If you're tasting that, oxidation is at least part of your problem.
I'm probably going to sound snobby, but I'd put the Mr. beer setup on craigslist and get better equipment. You an go surprisingly cheap if you go BIAB, shop around some, score used equipment off craigslist, and/or enjoy doing a bit of diy. Great beer may take some extra equipment (and practice), but pretty darn good beer can be made with the basics and some attention to detail. If you don't have one, get yourself a fermentation vessel that seals well and has an actual airlock. Plastic, glass, whatever. You have options. Some are better than others. Glass is probably better in the long run, but a plastic fermentation bucket will do the job too. I'll say it again, scrap the Mr. beer fermentation vessel.
Second, patience. You waited 3 weeks to bottle, so you have more than enough patience. I'm going to suggest that as a beginner, you are far far better off not checking the progress of the fermentation. Just don't. Every time you open the lid, you introduce oxygen, risk contamination, and reduce the quality of your final product. And yes, repeatedly opening the container to check the S.G. absolutely can have a huge affect on the flavor, particularly when your equipment and technique are suboptimal (as they almost certainly is at this stage). Check your O.G. and your F.G. and that's it. When you get better with your technique and have better ways to check the progress of your fermentation, then you can consider it again. But for now, just stop it. Your eyeballs and the activity in the airlock will tell you it's fermenting and when it ain't. Wait a week more and bottle it. Don't open the fermentation vessel until you are confident that it is time to bottle. Make sure you're using a decent fermentation vessel and resist the temptation to keep cracking it open to check on it and I expect that will fix half your issues.
Third, fermentation temperature matters, but it is unlikely to be you're biggest issue when you're starting out. You almost certainly have one or more major (though possibly simple) issues that are dwarfing any temperature effect on flavor. You've got a way to hold a temp and that's great, so don't stop. But, you'll notice that none of the comments have focused on temperature. As long as you're avoiding the extremes, worrying about temperature comes later as you get more skilled.
Fourth, I'll just echo the comments on expiration dates. I have nothing to add there. Kits and cans of extract can sit on the shelves for a long time, if the shop mostly serves experienced home brewers or they've been shuttered from the pandemic.
Fifth. Water matters and just going by taste isn't really a good way to go. I suspect this is where the other half of your problems are. Find a reasonably comprehensive water chemistry report for your specific water source. Virginia Beach, presumably, but figure out where your water actually comes from. Water chemistry reports are almost always publicly available, though they aren't always detailed enough to tell you what you need to know. If this is your's (
https://www.vbgov.com/government/de...ities/water-quality/Documents/2021CCR_Web.pdf), it doesn't really have all the information you'd like and seems to just be reporting ranges and averages and is missing some useful information. So, find a water chemistry analysis for your local tap water. Then, go online and find yourself a water chemistry calculator. I usually use the one on brewersfriend, but there are plenty out there. Compare what you have against what sort of water profile is the closest match for the style of beer you're going for. It really can make a huge difference. I had a similar issue in my first few brews and fixing my water is what made the difference for me. If you can't find a water chemistry analysis that contains the information you need to plug into the calculator, then consider building your water from the ground up with RO or distilled water and the appropriate salts. You mentioned your filtered tap water and the Walmart spring water tasting the same. There's an excellent chance that they are one and the same. An awful lot of bottled water is just filtered tap water, no matter what they call it. If you can find a local homebrewer that knows what they're doing, they may be able to tell you what the right fix for your water is. Mine just needs some gypsum. Your's? Who knows.
Last thought: If you can, find someone in your area that brews and watch them brew or (preferably) have them watch you. That includes bottling. You may be doing something incredibly simple to to fix, but it's a little hard to diagnose from a distance. There are many simple ways to screw up the flavor of what could have been a perfectly good beer. You may be doing something that you wouldn't even think to mention and we'd never think to ask about. If you can have someone watch you brew that knows what they're doing, you may save yourself a lot of headaches.