Sorry if this is going to sound harsh, but nobody makes 7 digits in 3 years by asking for things on a plate. Also, statistically speaking, you aren't going to just stumble upon a gold mine.
Your best bet is to close your browser, get off your chair and go start something that resembles what you see in profit 100/200 websites.
Don't waste time with stupid Bitcoin/IPO-scam/whatever-the-latest-get-rich-quick-fad-you-see-on-hn me-too crap.
I think this should be an automated response to anything on HN about "how do I make $X in Y time"
If I can think of a way to make 2.5mil in 3 years, I guarantee you I will not tell you, I will do it right here and now.
So first lets ask why do you even need 2.5mil in 3 years, why the urgency? Did you borrow money from a bookie? If that is the case I would recommend getting to a bigger bookie to create a turf war. Then while they are at it sneak out of the country and live under an alias somewhere in rural china for the rest of your life.
Prostitution? But even that isn't going to hit $2.5 mil in three years. More likely, you'll need to make all 2.5 mil in your third year, and spend the first two building yourself up.
You could try doing some tricky loophole pyramid scheme stuff using cryptocurrencies like Bitcoin? But that's also pretty scummy.
I don't think you can get more than 100k for one, and even that sounds high. You'd have to find 25 people with two working kidneys, which is almost definitely going to take more than three years (and a lot of ice).
You are asking the a question to achieve a preconceived and, IMHO, wrong solution. If you achieve your solution, you will very likely find the solution is the correct answer to your question (how do I avoid work), but is the wrong solution (no happiness).
The correct goal is not the avoidance of work, but the enjoyment of life.
Your proposed goal has a lot in common with winning the lottery: work for 3 years (buy lotto tickets), accumulate (win) $3MM, then never have to work again.
Looking at people who won the lottery, we find many of them go on to have an unhappy life: