Sorry for your loss / congratulations on your inheritance.
Certainly that money can last a lifetime. There are high yield savings accounts that will generate 3% a year or more in interest. You could live off the interest only, leaving the $3 million principal for your kids to inherit when you die. On 3 million, 3% interest is in the neighborhood of $100,000 per year, forever. You would be living the lifestyle of someone who earns $100k (plus anything extra you earn). Search for 'high yield savings account' and you should find some good places to put your money.
An investment like that is risk-free, but the rewards are relatively small -- $100k per year won't get you your own jet. You *could* turn 3 million into much, much more, but not without the risk of losing it. It's wise to have a diverse portfolio of investments with a mixture of low-risk and high-yield holdings. For as much money as you have, some professional financial advice would be worth paying for here.
Most importantly, think long and hard about your risk tolerance. Living off this money forever is possible, but you can't live like a rock star then. There are higher yield investments that will let you live much better, but they are gambles, literally. You can lose big. But if $100k per year is enough for you, you can generate that forever with zero risk and *still* have the 3 million to leave to the next generation. Just tell your bank manager that that's what you want to do and they'll set you up.
Best of luck,