So, the choices that you have is
1) Purchasing and passively owning some bitcoin. If you do this, it would be imprudent to trust anyone besides yourself to safekeep your bitcoins.
2) Creating some bitcoin related business.
The difference between 1 and 2 is night and day. I would expect a bitcoin related business to be an enormous hassle in terms of time commitment required. The time drain factors are regulation and money laundering hassles, thieves trying to steal your bitcoins, plus other factors inherent in any other new business venture.
So, if you want to put that 3 million to work as part of your big portfolio, it may be disproportionately time consuming to do it as a bitcoin related venture.
Realistically, this leaves the passive option as the only one viable.
I do not have an intelligent opinion on where the bitcoin prices are headed, and neither does anyone else. So please do not interpret my reasoning as a suggestion that owning bitcoins is better than owning an index fund. I am just saying that more active options may not be a good choice for a "$3M portfolio investment".
The last choice, passively investing in a bitcoin business, is only tenuously related to bitcoins and is akin to any other venture investing.
I agree with hheelll. Take that 3 million and invest in a related business. Forget about the mining business. Mining hardware manufacturers have no incentive to sell you the hardware early on when they could have mined with their brand new, super powerful hardware, and then sell it to you for high margin profit. Basically you are getting hand me downs. Once you acquire the hardware, the difficulty increase 15%-20% average every 12 days. Meaning the capital investment you just made lost that much in value every 12 days.
Someone said to deploy 3 million dollars worth of hardware and ROI in 10 days. Can someone ship that much hardware within reasonable amount of time? Can you deploy that much hardware within reasonable of time? BFL and KnC (miner manufacturers) can do the same thing and beat you to the punch.
A better solution is to take the 3 million and setup shop where you can have cheaper electricity. Spend some of that 3 million on solar and wind power to lock in power cost. Utilities have a tendency to increase power rates by 3% average everywhere. If Bitcoin flops and no one wants to have miners in data center, the data center can host some other equipment.
The ultimate money maker is to own an hydroelectric power plant. It is literally like printing money.