If you’re aiming for 3 million yen in one month, the only option is high price × leverage
Honestly, there isn’t truly an 'easiest' option. Realistically, with a balance of decent reproducibility and relatively high leverage, the practical choices are: 1) act as a sales agent or referral sales for existing high-ticket offerings (not investments, but B2B services, etc.), close 1–2 deals and collect a performance-based commission; or 2) turn your expertise or experience into a product priced at 100,000–200,000 yen as a 'one-month limited online course' and pre-sell it to about 30 people. Both approaches require riding on something that’s already selling or on a proven strength of yours; otherwise hitting 3 million is almost impossible. So first, decide on one thing you’re capable of moving other people’s money or time for, go all-in on it, and picture staying in sales mode for just one month.