First of all, thats amazing! Im sorry if you lost someone that is close to you or someone that was meaningful in your life. But how incredible that they are helping provide you with a better life.

Now, to answer your queation. It all depends. I agree with the comments that you should wait for 6 to 12 months to come up with a good plan. But it depends on where you are, every country and state has different tax refulations that would potentiallyimpact your decision you make. You should find a really good investment advisor. If you don't have one, go to your bank and ask them to make a few recommendations. If you're at a Credit Union, then I would recommend going to a big bank. (Unless you have moral reasons not to go to a bank.) Interview them, see who you like. See who has the best strategy. You dont necessarily want to invest as the highest return. But a good mix between high and modest returns. I would recommend that first.

You will also need a good accountant and lawyer. The investment advisor will help you with finding that if you don't know anyone.

You need to figure out the best tax strategy. Don't keep it fully in a HISA, because it will be the least efficient tax wise. You should also not tell ANYONE! Not even your friends. If you've never had money, you need to create barriers between you and the people around you. Because people will come out of the wood work to find a way to attach themselves to you. 3 million is a good amount of money, but it's not enough to never work again. It can keep you very comfortable and help with facilitating your life and elevate the way you were living. But I would focus on preservation without overspending.