Investing in 2020 has been challenging, to say the least. Wall Street has arguably been taken on its wildest ride in history, with the widely followed S&P 500 losing 34% of its value in under five weeks, then regaining all that was lost in less than five months. The steepness of the decline and swiftness of the rebound represent all-time records.

Despite this heightened volatility, we know that, historically speaking, big declines in the stock market are always buying opportunities. Eventually, every stock market crash and correction in history is firmly placed into the rearview mirror by a bull market rally.

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But not all investors feel comfortable investing in individual stocks or have the time to devote to researching companies. That's where exchange-traded funds (ETFs) come into play.

An ETF allows investors to buy a targeted basket of stocks based on market cap, industry, growth/value, region, or a host of other factors. The point is, ETFs make life a lot easier for folks unwilling to buy individual stocks, and they do so for a reasonably low annual fee (the net expense ratio).

Keep in mind, though, that ETFs don't have to be boring substitutes for individual stock ownership. Just because you've decided to buy a security that's effectively a basket of stocks, it doesn't mean you have to forgo substantive growth opportunities.

Furthermore, growth stocks are primed for success with the U.S. economy struggling. The Federal Reserve has pledged to keep interest rates at or near record-tying lows for the next couple of years, which means high-growth companies are going to be able to borrow very cheaply.

If you have, say, $3,000 at your disposal that won't be needed to pay bills or cover emergencies, then you have more than enough to buy into the following three high-growth ETFs.

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Global X Cloud Computing ETF

If the coronavirus disease 2019 (COVID-19) pandemic has taught us anything, it's just how important cloud computing and enterprise cloud sharing is going to become in the years that lie ahead. We were already witnessing a pretty steady shift toward increased cloud demand before COVID-19, but the pandemic has provided a shot in the arm of organic growth that should be long-lasting.

The high-growth ETF of choice here is the Global X Cloud Computing ETF (NASDAQ:CLOU), which has 36 holdings focused on infrastructure-as-a-service, payment-as-a-service, software-as-a-service, cloud-edge computing, and hardware. Its net expense ratio of 0.68% is a bit high, but it's expected to be a somewhat actively managed fund with a number of cloud-focused initial public offerings coming to market. 

Of the Global X Cloud Computing ETF's $1.15 billion in assets under management, Zoom Video Communications is by far the largest holding at 11% of assets. Other well-known top-10 holdings include Twilio, Shopify, and salesforce.com.

To offer some context, Zoom, Twilio, Shopify, and salesforece.com are on track to deliver respective sales growth of 286%, 41%, 65%, and 22% this year. This is the perfect way to take advantage of an industry that isn't offering much in the profitability column yet, but is growing exceptionally fast. If one or two cloud companies fail to deliver, you won't be penalized because you'll own stakes in the many infrastructure, payment, and software service plays that do thrive.

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First Trust Nasdaq Cybersecurity ETF

Speaking of rapid technology-based growth, another ETF just begging to be bought is the First Trust Nasdaq Cybersecurity ETF (NASDAQ:CIBR). With the traditional office environment completely disrupted by the coronavirus pandemic, remote and shared-work environments have taken on greater importance. This has firmly put cybersecurity companies in the spotlight.

The First Trust Nasdaq Cybersecurity ETF currently has 41 holdings and a net expense ratio of 0.6%, which'll be partially offset by a very small yield from a handful of dividend-paying stocks. As the name implies, this ETF seek to buy companies involved in the cybersecurity segment of the technology and industrial sectors. 

As of Wednesday, Sept. 23, CrowdStrike Holdings (NASDAQ:CRWD) and Okta (NASDAQ:OKTA) were the two largest holdings, comprising 13.1% of invested assets on a combined basis. What you find with the likes of CrowdStrike and Okta is that their businesses are designed to grow with their existing clients. In CrowdStrike's latest quarter, it announced that 57% of its customers had four or more cloud module subscriptions, up from 9% a little over three years ago.

Meanwhile, Okta's identity verification platform is entirely scalable and designed to be added on as businesses grow. As of the most recent quarter, 95% of Okta's revenue was derived from transparent, high-margin subscriptions. 

These are two consistent double-digit growth companies that are highly representative of what you'll get with the First Trust Nasdaq Cybersecurity ETF.

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AdvisorShares Pure U.S. Cannabis ETF

Another high-growth industry with plenty of near-term risk but substantive long-term growth and reward potential is marijuana. Unfortunately, most cannabis ETFs tend to lump in Canadian pot stocks, which have been an absolute train wreck to this point. That's why the first all-U.S. pot ETF, the AdvisorShares Pure U.S. Cannabis ETF (NYSEMKT:MSOS), should be on investors' buy lists.

Consistent with other marijuana ETFs, investors in the AdvisorShares Pure U.S. Cannabis ETF can expect to pay a 0.74% expense ratio. That's not cheap, but it's also not unreasonable for an actively managed and recently introduced ETF whose sole focus is on a country where legalized weed sales could triple to as much as $37 billion by 2024.

The four-largest holdings are Curaleaf, Green Thumb Industries, Innovative Industrial Properties (NYSE:IIPR), and Trulieve Cannabis (OTC:TCNNF), all of which range in weighting between 7.9% and 8.9%. Innovative Industrial Properties and Trulieve Cannabis are particularly noteworthy, as they're the most profitable pot stocks on the planet right now -- IIP on a per-share basis and Trulieve based on nominal operating income. 

Even if marijuana were to remain illegal at the federal level for many years to come, new state-level legalizations and organic growth in already legalized states offers more than enough upside for U.S. multistate operators and ancillary players. The U.S. is the smart way to invest in cannabis, and that's exactly what the AdvisorShares Pure U.S. Cannabis ETF will allow you to do with your $3,000.