My original thought was this was just free money without taxes, but even with taxes it is decent.
If you received $3M, by my estimates you would pay around 25% in income taxes. That leaves you with $2.25M.
The safe withdrawal rate where you can withdraw from an investment without reducing the principle is generally calculated at about 3.5% per year.
3.5% of $2.25M is $78,750. That is certainly enough to live off. For the record, the number is $90K if you don't have to pay income tax on the $3M.
Now when you add in a person's existing savings that could easily mean they never have to work again. I know there are probably rich CBers who would balk at $78K per year but for the majority of people that is a completely life-changing amount of money.