Investing in growth companies can be a smart and effective portfolio-building strategy. While their shares will experience fluctuations in both directions during shorter time frames, as long as the underlying businesses continue to grow, the value of these investments should follow a long-term rising path. And when the prices of growth stocks tumble, those can be opportune periods to load up on them.

If you've got $5,000 that you can afford to invest for the long-term right now, three stocks you should consider are Oscar Health (OSCR -4.04%), Roku (ROKU -3.09%), and FedEx (FDX -1.23%) -- all of which have been falling in recent months.

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1. Oscar Health

Oscar Health is a health insurance company that differentiates itself through its use of technology. It offers a concierge team to its customers, who can also use its app to get answers to common questions and to review and manage their benefits.

The company reported its second-quarter results on Aug. 12. Total premiums earned in the period soared 364% year over year to more than $528 million. That was thanks in large part to a 35% in growth in membership, as Oscar Health reported 563,114 members as of the end of the period, up from 417,480 a year earlier.

However, despite this impressive growth, the healthcare stock is still down by 29% over the past three months (during which time the S&P 500 has gained around 5%). The notable blemish on its last earnings report was a loss of $50 million on adjusted earnings before interest, taxes, depreciation, and amortization (EBITDA) -- a larger loss than it booked in the prior-year period -- due to pandemic-related expenses, including higher administrative costs. But with vaccination rates in the country increasing, the impact of that particular headwind should ease. And an improvement on the profitability front could generate some bullishness around the stock.

Oscar Health may not be a popular stock right now, but it presents loads of potential. It has partnered with Cigna Health to offer co-branded health insurance plans that will allow it to reach more consumers. It has a presence in just 18 states, so there's still plenty of room for it to expand its operations. And by leveraging technology and catering to a potentially younger (and healthier) member base, it could deliver some solid financial results in the future. 

2. Roku

Roku gives investors a way to tap into the growing popularity of streaming services. Not only does the company have its own content on The Roku Channel, but its streaming devices and platform make it easy for consumers to consolidate all their subscriptions into one place.

Enthusiasm for the stock may be waning as investors anticipate that the companies that benefited during the pandemic's lockdowns will see their businesses falter somewhat as consumer behaviors return to normal. Roku shares are down 8% over the last three months.

However, the second-quarter results don't much support that pessimistic view about Roku. The company's active accounts totaled 55.1 million -- up a modest 2.8% from the 53.6 million accounts it reported a quarter earlier. While streaming hours did fall sequentially from 18.3 billion in Q1 to 17.4 billion, on a year-over-year basis, they were still up by 19%. And it's the company's platform revenue (which includes advertising and content distribution) that is showing the most growth. Revenues for the segment rose 117% year over year to $532.3 million. By comparison, revenue from its player segment (i.e., hardware) rose by just 1%.

While it's good for the company when consumers buy Roku devices, those are not likely going to be recurring purchases. The platform segment, by contrast, presents much more potential over the long term. 

Roku's stock has cooled off of late, and now may be an optimal time to buy in -- it hasn't been this low in months. The rise of streaming services seems unstoppable, and given Roku's growing user base in that expanding segment, this could be a top growth stock to hold for many years.

3. FedEx

Logistics company FedEx also looks to be a safe growth buy for the long haul. The pandemic has led to a surge in e-commerce activity across the world, both for traditional retailers and online-only operations. Plus, platforms like Shopify and Etsy have made it easy for individuals to make money by selling their crafts online. While there will be an inevitable decline in e-commerce activity as people feel safer about returning to brick-and-mortar stores, doing a greater portion of one's shopping online may be part of the post-pandemic new normal for many people.

So even though FedEx experienced unusually strong business growth over this past year, investors shouldn't be quick to assume that the growth is about to suddenly come to a halt. In the company's fiscal 2021 (which ended on May 31), its sales grew by an impressive 21% to $84 billion. But Chief Financial Officer Michael Lenz expects more growth ahead: "We expect continued strong momentum in fiscal 2022, and our investments are focused on the areas of greatest growth and highest returns, like e-commerce, to position us for sustained long-term growth in earnings, cash flows, and returns."

One of the biggest improvements for the company was on the bottom line: Its net income of $5.2 billion was quadruple the $1.3 billion profit that FedEx reported in the previous year as the company grew its top line while keeping its expenses relatively steady.

Continued growth in online spending -- analysts believe retail e-commerce sales will rise by nearly 17% this year -- combined with a focus on improving operations and making them more efficient should lead to even better results for FedEx over the long haul. Investors should take advantage of the stock's 13% drop over the past three months, as that dip in its value looks unlikely to persist.