In theory the things that would make the "as much money as possible" in such a short amount of time would be some form of gambling. That could be traditional gambling, options, illicit activities (gambling that you escape the law), etc. However, these tend to have a low probability of working out.

So I think we have to define what level of probability you are looking for. Sure thing? Then maybe consulting. Possible with some follow-up work? Then maybe an app, SaaS, etc. High payout but unlikely? Then some type of gambling - options, crypto, traditional gambling, etc.

Consulting is often the quickest cash, assuming you're an engineer with a decent network. You're not on socials, but do you have people you've worked with? Contacts at businesses? Other devs?

Email everyone telling them you have a few weeks free.

You could also call tech recruiters who could potentially place you on a short gig fast. But will naturally take a cut.

Alternatively, I know others made good money doing crypto day trading, but I'd imagine you'd have to know what you're doing.

Criminal activity, depending on your skill set.

Commit fraud, launch a phishing campaign, sell drugs, buy bulk SSN and PCI info off the dark web. Hell ransomware a hospital if you’re desperate and have no soul.

Three weeks isn’t really enough time to find legitimate development work unless you have prior contracting relationships with folks who frequently need short term projects outsourced.

It’s a little easier if you have some security skills. Lots of banks or regulated industries get way behind on things like PCI audits and contract out that work close to deadlines.

Consulting, lecturing or writing on hire. Also, selling owned goods such as out-of-print books, clothes, electronics. Joined with this, drastically reducing expenses during these three weeks (no extra spending, switching to one meal a day, etc.)

With no setup, no network, no products or existing business? Probably a freelancing website, or finding local businesses that need help creating a website. Try contacting local agencies to see if they have any work they need help with.

Meaningful amounts of money require existing resources. It's like losing weight - you have to do it over time.

Drugs. Internet advertising. Pyramid schemes (lularoe/amway/cryptocurrencies/web3). Muggings. Gambling.