The great thing about living in the United States is that we have virtually unlimited choices. (Proof: Have you ever checked out the potato chip aisle?) The same holds true for the cornucopia of stocks you can invest in. You can purchase more than 3,300 stocks on The Nasdaq exchange, while there are an additional 2,800 up for grabs on the New York Stock Exchange.

With so many choices, it's hard for investors to choose which stocks are the best to invest in. We asked three successful investors and Motley Fool contributors what they'd choose if they could select only three. The answers may surprise you.

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Barbara Eisner Bayer: Diversification is a key component of any successful portfolio. It reduces risk but gives you the opportunity to beat the market with a few big winners. It's hard to incorporate diversity into a portfolio of just three stocks, but I'm going to try. The most important factor to me is that they're all excellent businesses with strong management. But in order to diversify, I'm going to choose companies that have very little to do with each other.

My first choice will be the heart and soul of my three holdings -- and that's Berkshire Hathaway (BRK.A 0.49%) (BRK.B 0.35%), led by the incorrigible and brilliant Warren Buffett, who recently turned 90. Owning BRK is like owning the greatest mutual fund out there -- except it has arguably the best manager ever at its helm and no management fee. The company is less like a stock and more like a holding company for some incredible businesses, including GEICO, Dairy Queen, Fruit of the Loom, and Duracell.

And since Buffett knows he can't live forever, he's empowered his lieutenants Todd Combs and Ted Weschler to purchase stocks in their own style, which has led to recent purchases of Apple, Amazon (AMZN -0.32%), and most recently, the IPO Snowflake. That's in addition to its bank holdings like Wells Fargo and Bank of New York Mellon, and such stalwart companies as Coca-Cola, Kraft Heinz, and General Motors.

By owning Berkshire, I'm free to include a more risky, high-growth stock in my three holdings, and my choice is DocuSign (DOCU 1.31%). Anyone who has needed to sign an important document during the coronavirus pandemic is no doubt familiar with the company, which enables people to deliver their John Hancocks online and close deals no matter where they are in the world.

As of Sept. 1, shares were up 190% in 2020 after surging almost 85% in 2019. And last year, the company created the "Docusign Agreement Cloud," which enables any business to automate an entire agreement life cycle. The stock may be a bit pricey now, but I believe it will grow into its valuation over the long term.

For my last choice, I want a company that's going to keep giving me free money. So I'm going to choose a dividend stock. But instead of looking at Dividend Aristrocrats, I'm going to select a real estate investment trust (REIT) in a growing industry -- marijuana -- and choose Innovative Industrial Properties (IIPR 2.83%).

The company buys the land under a legal cannabis grower and then leases it back to the seller. Since the company is a REIT, it must return 90% of its taxable income to shareholders -- hence, the healthy dividend, which most recently was around 3.2%. IIRP nicely rounds out my three-stock portfolio and will guarantee that no matter what happens to my other two holdings, I'll always be receiving income.

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Eric Volkman: For my trio, I'd want two that are still relatively young companies with enormous potential still in front of them. As to the third, I'd be aiming for a steady business that spits out a regular, nicely yielding dividend.

In the former category is Square (SQ 0.22%). Since none of us are going out much these days, it's easy to forget that the company's distinctive payment registers and terminals are becoming ever more prevalent. These point-of-sale devices, meanwhile, are hooked onto an ever-mushrooming ecosystem of complementary goods and services, giving the company enormous scope for expansion.

Stock No. 2 is Facebook (META 2.23%). Despite persistent controversy over numerous issues and constantly rising competition for eyeballs, it's still far and away the social media company on the planet. It's hard to beat its powerful one-two punch of advanced ad targeting and enormity of user base, which makes it a must-spend for a great many advertisers.

I don't see anyone topping Facebook in that combination of factors, so I fully expect the company to stay dominant and growing for years.

Finally, my pick for the "Steady Eddie" of the bunch is Digital Realty Trust (DLR 0.25%). This veteran real estate investment trust (REIT) is one of a handful that specializes in data centers -- vital facilities for the digital age. The company operates data centers around the world.

More are coming. A recent estimate has it that roughly 65% of all corporate-server computers are still located at their company's facilities. So the number of potential new clients is immense. With its presence and its experience, Digital Realty Trust feels like the data center REIT best positioned to take advantage of this. And its yield, while not the highest in its niche, is still quite substantial at 3.2%.

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David Butler: If I could only own three stocks, I'd have a portfolio based on growth countered by a value pick.

First and foremost, I'd focus on American companies. There's no need to flirt with foreign equities in a three-stock portfolio, and the U.S. economy is still the best to do business in. The stocks themselves would come down to three attributes: Broad economic potential, and a combination of growth and value. Nothing crazy, as the lack of diversification means you can't take as much risk on anything.

It's a truism, and it's borderline boring, but the cards are simply stacked in Amazon's favor right now. In a three-stock portfolio, I'd take it. The e-commerce company has such a demonstrated control of the retail market, and social distancing has only enhanced the company's positioning. Structurally, there just doesn't seem to be anything at present that will stop the story of what Amazon has been doing to the retail sector.

Second would be a financial play. Banking is one of the few sectors that isn't commanding huge premiums and offers deals for those who can buy and wait. Low interest rates or not, this is an important piece of any portfolio. My choice would be JPMorgan (JPM 0.24%). Prior to 2020, the bank had excellent returns on equity, and Jamie Dimon has done an incredible job keeping the bank at the head of the pack. The 3.9% dividend doesn't hurt either.

The final has to be Berkshire Hathaway. The conglomerate carries the vast insurance assets built by Warren Buffett and has nearly $150 billion in cash on hand to invest at its leisure. That capital and quality of assets make it a compelling play that will be able to get in on any game it wants -- even after Mr. Buffett's time at the helm expires. By owning shares, you'll get in on those plays, too. This is a valuable counter lever to the more speculative pricing that one pays to invest in Amazon.