For more than a year, volatility has been a way of life for investors. The uncertainty brought about by the coronavirus pandemic ultimately sent the CBOE Volatility Index, which measures the perceived volatility in S&P 500 (^GSPC -0.88%) options over the coming 30 days, to an all-time high in March 2020.

Even though volatility is almost always an excellent opportunity for long-term investors to put their money to work in great companies, some investors prefer to keep their distance from the market's wild gyrations. That's where exchange-traded funds (ETFs) can come into play.

An exchange-traded fund is a security that trades like a stock but holds a basket of stocks or bonds all at once. It offers a number of advantages, including instant diversification that can, in some cases, rival mutual funds, as well as the liquidity to buy and sell on an as-needed basis. Additionally, the diversification provided by ETFs can make them less volatile than owning individual companies, which may appeal to more conservative long-term investors.

Owning a handful of high-performance ETFs covering the right trends is all an investor may need to get rich. The following three ETFs are in the sweet spot of consistent growth trends and should offer plenty of long-term upside for patient investors.

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Vanguard S&P 500 ETF

I never said I was going to win any points for originality. The Vanguard S&P 500 ETF (VOO -0.84%) might be about as basic as it gets, but this is a proven moneymaker for set-it-and-forget-it investors.

As its name implies, the Vanguard S&P 500 ETF is a tracking index that attempts to mirror the performance of the benchmark S&P 500. The goal here isn't to beat the market with this ETF, but to effectively match its performance over time. Vanguard buys stakes in the 500 companies (some of which have multiple classes of shares) that make up the S&P 500, with the ETF very closely following the movement of the widely followed index.

Why the Vanguard S&P 500 ETF? For one, the net expense ratio -- i.e., what you'll pay in fees -- is a mere 0.03% each year. For every $1,000 you invest, you're only going to lose $0.30 to fees, which is pretty exceptional considering the diversification you're being afforded.

Perhaps an even more compelling reason to buy is its long-term performance. Crestmont Research recently examined the S&P 500's 20-year rolling total returns (including dividends) for every year between 1919 and 2020. The result? No matter when you invested in the S&P 500, you generated a positive average annual total return as long as you held for 20 years. In fact, only two of the ending years (1948 and 1949) produced average annual total returns of 5% or lower. Comparatively, over 40 end years would have produced a double-digit average annual total return.

There's nothing glitzy whatsoever about the Vanguard S&P 500 ETF. But with the benchmark S&P 500 averaging an 11% total return since 1980, it's a genius way to get rich.

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ProShares Pet Care ETF

Now, if you want an ETF that has a really good shot at outperforming the broader market over the long run, consider buying the ProShares Pet Care ETF (PAWZ -0.30%). While this ETF seeks to mirror the performance of the FactSet Pet Care Index, the gist is that you'd own businesses in the U.S. and globally that are engaged in caring for companion animals, such as cats and dogs.

According to data from the American Pet Products Association (APPA), it's been at least a quarter of a century since year-over-year U.S. expenditures on pets declined. This year alone, nearly $110 billion will be spent on our furry friends in the U.S., with an estimated $44.1 billion for food and treats, $32.3 billion for veterinary care, and $23.4 billion for a variety of supplies. 

If the above isn't convincing enough, also consider that the number of U.S. households which owns a pet has increased from 56% in 1988 to 67% by the 2019-2020 APPA survey. Pets are like family, and owners will spend big bucks to ensure their happiness and well-being.

The ProShares Pet Care ETF has 27 holdings (mostly from the U.S.), five of which make up around 46% of all assets. One of these core holdings, Freshpet (FRPT 0.82%), is up tenfold in just the last four years. Freshpet's focus is to produce organic and natural food and treats for dogs and cats. Similar to how the organic food craze for us humans powered grocery sales higher in the mid-2000s, Freshpet is seeing huge demand from owners who want higher-quality food and treat items for their furry family members.

Because it's a considerably smaller ETF (by assets) than the Vanguard S&P 500 ETF, you shouldn't be surprised to see its higher net expense ratio of 0.5%. But taking into account the near-guaranteed growth potential in pet care, it's a small price to pay for quality diversification. 

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AdvisorShares Pure U.S. Cannabis ETF

If supercharged growth is more your thing, the AdvisorShares Pure U.S. Cannabis ETF (MSOS -5.30%) is what you're looking for. As the name implies, this is an ETF that purchases marijuana stocks focused on the U.S. cannabis market. 

Although marijuana is a big-time growth trend globally, the U.S. is the epicenter of cannabis profits. New Frontier Data has forecast an annual average sales growth rate for U.S. cannabis of 21% between 2019 and 2025. If accurate, the pot industry should be generating about $41.5 billion in annual sales by mid-decade. Considering all the regulatory issues plaguing Canada, Mexico, and parts of Europe, the U.S. shouldn't have any trouble showing pot stock investors the green.

The AdvisorShares Pure U.S. Cannabis ETF is predominantly comprised of multistate operators, or MSOs. An MSO is a vertically integrated company that handles the seed-to-sale process in legalized states. Since interstate transport isn't legal with marijuana a Schedule I substance at the federal level, MSOs often grow, process, and even sell their own cannabis.

The largest holding, by just a few hundredths of a percent, is MSO Curaleaf (CURLF -2.56%). Curaleaf has more operating dispensaries (106) and retail licenses than any other MSO. All told, it could open more than 130 dispensaries across 23 states. Following its acquisition of privately held MSO Grassroots and the Select brand of pot products last year, Curaleaf should be the first pot stock to hit $1 billion in full-year weed sales.

With the green rush here to stay, a 0.74% net expense ratio is a reasonable price to pay for diversified growth in the hottest marijuana market on the planet.