Maintaining a portfolio of multiple stocks is just a smart idea. Diversification reduces the risk of unexpected developments and provides more opportunities for upside while also limiting the overall impact of stock-specific setbacks. The Motley Fool recommends building your portfolio to eventually hold at least 25 different long-term stocks at a time.

On the flip side, most investors find that a small number of stocks end up becoming the core of a portfolio. These are the positions with strong, reliable performances that make them worth holding forever.

With that as the backdrop, here's a closer look at three such companies that could serve as the pillars of a million-dollar portfolio. Note that none of them manage complicated business models. They're just businesses that are perpetually in demand.

1. Microsoft

Microsoft (MSFT 1.29%) isn't just a software powerhouse -- the company's responsible for shaping the modern computing landscape. Its Windows operating system makes personal computers easier to use and helps users connect with the world, and its productivity software allows businesses and individuals to become highly efficient and organized. Both Windows and Office are just as necessary today as they've ever been.

Then there's the Microsoft that's not as obvious. This company is a key player in the cloud computing market, collecting 24% of last quarter's worldwide cloud spending of $62.3 billion, according to data from research outfit Canalys. Microsoft is also the name behind the Xbox video gaming console, owns professional networking platform LinkedIn, operates a search-advertising business, offers a line of high-performance laptops, and has monetized a whole host of other tech-based solutions to meet highly specialized business needs.

Yes, the company is big and old. It's hardly on an intercept course with obsolescence, though. If anything the world is more reliant on computers than it's ever been, playing right into the hand Microsoft is holding. In this vein, Microsoft has not reported a year-over-year dip in quarterly revenue since the latter half of 2017. Indeed, not once during that stretch has quarterly revenue failed to grow less than 12%.

Don't look for this growth streak to come to a close anytime soon, if ever. Microsoft is slowly but surely evolving its products to generate recurring revenue. While the monthly fee for cloud-based access to software like Word or its cloud-computing platform, Azure, is nowhere near the sort of revenue produced by an outright sale of this software, the subscription model effectively ensures these customers pay their nominal monthly fee in perpetuity. This predictable revenue stream is a very big deal. The company simply has to focus on bringing new customers to the table.

2. Medtronic

If you find it in a hospital, there's a good chance Medtronic (MDT -1.25%) made it. Defibrillators, ablation systems, patient monitoring tools, surgical implements, stents, and dialysis machines are just some of the wares in its wheelhouse. And many of its products are highly specialized. Its LigaPASS 2.0 surgical robot, for instance, was recently cleared by the Food and Drug Administration (FDA) to perform ligament augmentations in spinal surgeries; it's the only device authorized to perform such a procedure.

Such a deeply diversified portfolio can be a double-edged sword. Medtronic's revenue -- and earnings -- are relatively steady from one quarter to the next. But this many different products makes it difficult for one hit product to make a significant impact in any given quarter, or year; its successes are diluted just as much as its missteps.

If you can tuck this stock away for the long haul, though, it's worth the wait. Shares are worth more than twice what they were worth 10 years ago, and that's factoring in the stock's 34% slide since September's high. The quarterly dividend's grown from $0.24 per share then to $0.68 per share now as well, bolstering the stock's net returns whether or not they were reinvested.

Would-be buyers may not want to tarry either, if they're interested. While this is a ticker with a history of volatility, it's also got a reliable history of quick, sizable rebounds from its sell-offs. There are just too many ways for Medtronic to drive revenue for investors to let the stock fall too far, or for too long.

3. JPMorgan Chase

Finally, add JPMorgan Chase (JPM 1.21%) to your list of tickers that could help you retire a millionaire.

Bank stocks are admittedly tough to be excited about owning now. Although the Federal Reserve has raised the federal funds rate by 150 basis points just over the course of the past couple of months, interest rates are still near historical lows, crimping the profitability of lending. And now, whether or not it evolves into a full-blown recession, the economy is running into a headwind that could dial back demand for banking services.

There are two important, related nuances of banking stocks in general -- and JPMorgan Chase in particular -- right now, however, that investors can't afford to ignore.

One of these nuances is that the bulk of any bad news is already priced in. Thanks to the stock's 35% pullback from last year's highs, you can step into JPMorgan shares at a mere 10 times this year's projected per-share profits, and less than nine times next year's expected earnings. The market's pricing in an apocalypse that isn't likely to pan out nearly as bad as feared.

The other nuance is, much like Medtronic's, JPMorgan Chase's stock has a history of bouncing back from pullbacks, since the big bank readily adapts to ever-changing circumstances. The bank bounces back from each cyclical lull, in fact, to become bigger and better than it was before. To this end, the stock's worth three times what it was 10 years ago, in step with its profit growth as well as its dividend growth.

As long as consumers need a means of borrowing money and building wealth while corporations require credit and capital, JPMorgan Chase is a name worth buying on dips, and worth holding on to through downturns.