I would hazard a guess that most of those who have responded here, do not have 3 million USD to invest in a speculative project.

Neither do I, however I (my family) is worth several million dollars, accumulated by earning, saving and investing, the old fashioned way. A part of my net worth is in liquid form and some of it is in form of real estate or capital equipment.

Consequently, I would not hold my breath awaiting an intelligent investment discussion here, especially on a topic as new and fluid as bitcoin investing. I hope that I am not insulting anybody.

On this sour note, let me continue and say that running bitcoin mining in an air conditioned data center would be prohibitively expensive for the following reasons:

1) You have to comply with various datacenter safety regulations. Proper electrical wiring for a high power density computing project is expensive. You may need to bring more power to your data center, pay for more utility transformers, etc.

While small miners or dedicated miners can skip a lot of safety gear, NEC compliant wiring, etc, an owner of a data center would have to follow all proper electrical practices required by authority having jurisdiction.

2) Assuming that your datacenter is cooled, the cost of energy required to mine, and to cool the air heated by mining, would be at least 3-4 times the cost of an un-air-conditioned miner installation.

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.