It's been a painful year for stock investors so far. Even after a huge day on March 2 that saw stocks gain almost 5%, the S&P 500 is still down more than 4% in 2020. Moreover, there are reasons to remain concerned as the COVID-19 coronavirus runs rampant across the world, causing economic harm as governments take drastic steps to halt the spread. 

Yet even in what has been a decidedly bearish start to the year, some stocks are delivering huge gains. Three of the best gainers in 2020 are Teladoc Health (NYSE:TDOC)SolarEdge Technologies (NASDAQ:SEDG), and Square Inc (NYSE:SQ), which have gained 48%, 35%, and 29%, respectively, so far this year. 

And as much as the three have been on fire to start the year, they also have wonderful long-term prospects. I'm not willing to predict what will happen over the months to come, but investors looking for multiyear winners should put these three on their lists. 

Man holding model rocket as it starts to launch.

These stocks have rocketed up to start 2019. They could shoot much higher in the years to come. Image source: Getty Images.

Shortening the distance between patients and care 

Teladoc's stock has been on an incredible run recently, in large part because of its remarkable growth rate, but also because, as a provider of technology that allows medical professionals to connect remotely with patients and that sounds just perfect in a world where COVID-19 coronavirus is spreading rapidly. 

Here's the thing: Teladoc's thesis isn't built on things like highly infectious disease. Sure, there could be some benefits for virtual care in those cases, but the big benefit for technology like Teladoc's is, simply put, making it easier and more affordable for people all over the world to get access to appropriate care more quickly. 

As an early mover in virtual care that's already connected to many of the world's biggest healthcare companies, Teladoc reported $553 million in revenue in 2019, a 32% jump from 2018. Healthcare providers used Teladoc to visit a massive 57% more patients last year. 

Here's the rub: The market's short-term interest in Teladoc could actually cause its shares to underperform -- or even lose value -- in the short term. The stock now trades for more than 16 times trailing sales, which is quite a premium to what investors have been willing to pay in the past:

TDOC PS Ratio Chart

TDOC PS Ratio data by YCharts

But even with the risk we see some "reversion" to prior valuation ranges, Teladoc is a well-established leader in one of the fastest-growing segments of global healthcare. Over the long term, it is primed to be a market-beating investment, particularly when it transitions from market-building (at the cost of profits) to profit-growing in the next couple of years. 

Short story: SolarEdge is crushing it

SolarEdge makes power electronics components that are used to connect solar panels to the grid, and finds itself in an enviable position right now. It, along with Enphase Energy (NASDAQ:ENPH), are by far the two biggest suppliers to the U.S. residential solar market, following some changes to U.S. building code that has pushed most other suppliers out of the market. 

As a result of this duopoly situation, business is growing like wildfire. Revenue increased 59% in the fourth quarter, and net income shot up 308%. Moreover, it looks likely that sales will continue to grow at a high rate, as new entrants to the U.S. residential market have been slow to materialize, despite a short-seller's thesis that another U.S. company with no experience in solar electronics manufacturing was going to disrupt the market in short order. Since the short attack was launched in September, the percentage of shares sold short is up more than double, but the company has continued delivering huge growth. 

That's not to say that there won't eventually be another entrant into the market. There almost certainly will be at some point because this is a high-growth market that needs multiple suppliers to thrive. But even with that likely to happen, the global growth opportunity for solar power electronics is massive, and growing.

Add in the company's ongoing efforts to expand into related fields like energy storage and electric vehicle power components, and SolarEdge should prove a market-beating stock for the long term, too. 

Square expanding its reach to more people and businesses

Square may have established its reputation by disrupting the merchant systems world with its affordable and portable credit card processing technology. The company has done enormous things to make it affordable and simple for more small businesses to accept cards for payments in more and easier ways. 

This has resulted in enormous growth, with shares up over 500% since the late-2015 IPO and over 300% in the past three years on the company's shift to profitability in 2019. Yet even after all this growth, Square is still a small fry in the world of financial services, with revenue of $4.7 billion. 

And that bodes well for continued growth for years to come. Square's growth in merchant services is on track to continue, and from businesses of all size, not just small mom-and-pops anymore.

But that's just the tip of the iceberg. Square is also pursuing a banking charter, which would allow it to vastly expand the services that it can offer its customers (though also adding substantial new regulation as well), and its Cash app, a peer-to-peer payments tool, has exploded in popularity and generated well over $1 billion in revenue last year from 24 million active users, including this author. 

Despite its enormous growth in the past five years, Square could be only just tapping its full potential. Like the other two stocks featured here, it is likely to be a volatile investment going forward, but its long-term prospects to deliver market-beating returns makes it worth buying now.