October wasn't a great month in the stock market, with investors' growing concerns surrounding the coronavirus pandemic leading to a late-month sell-off. Many sectors felt the impact, and many growth stocks are now trading at reduced prices.

Three stocks that were down more than the S&P 500 last month include Teladoc Health (NYSE:TDOC), Microsoft (NASDAQ:MSFT), and Mastercard (NYSE:MA). These companies still have lots of growth left in them, and now could be an opportune time to add them to your portfolio. Here's why you should be bullish about each over the long term.

1. Teladoc

Shares of Teladoc Health fell more than 13% in October, while the S&P 500 dropped by just 2%. However, this is still a great growth stock to invest in, especially now that its $18.5 billion merger with Livongo is complete.

Green arrows pointing up.

Image source: Getty Images.

The opportunities there are significant, as Livongo's focus on patients with diabetes and chronic conditions complements Teladoc's more generic telehealth services that aren't aimed at a specific disease or ailment. And with the companies sharing only 25% of the same customers, there isn't much overlap there yet, either, providing for some great upselling opportunities.

What's surprising about Teladoc's rough month is that the company ended it with a strong earnings report. On Oct. 28, it released its third-quarter results for the period ending Sept. 30, showing sales of $288.8 million, up 109% year over year. Its total visits also climbed to 2.8 million, which was a 206% increase from the prior-year period. And Teladoc's expecting a lot of that growth to continue, forecasting that visits could top 3 million in the fourth quarter.

With COVID-19 cases still on the rise, there could be even more demand for telehealth services in the future as people opt to stay home rather than visit the doctor's office, especially if there are lockdowns.

2. Microsoft

Microsoft's shares fell a more modest 4% during the month of October, and like Teladoc, this is another good stock to invest in right now if you're worried about the coronavirus pandemic. The tech company's products and services help allow businesses to connect easily and employees to work remotely.

The Washington-based business reported its results for the first quarter of fiscal 2021 on Oct. 27, showing revenue of $37.2 billion -- up 12% year over year -- and net income of $13.9 billion, up by an impressive 30%. Microsoft's cloud business, Azure, saw the most sales growth at 48%.

Most of the company's products and services -- including LinkedIn, Office 365, and Xbox content and offerings -- also saw greater than 10% growth during the period. This business is versatile and has adapted during the pandemic to the changing needs of businesses and individuals, whether they're remote workers, those looking for work, or just people sitting at home and playing Xbox.

The great news is that Microsoft is still growing, and that's likely to continue over the years as it evolves and develops new products. One of its fastest-growing segments is the laptop/tablet hybrid Surface, which showed 30% higher sales this past quarter. First introduced in 2012, Surface products have proven easy to use at home and on the go. They're a great example of the PC-maker's continued ability to innovate and uncover new growth opportunities.

3. Mastercard

Credit card company Mastercard rounds out this list of impressive growth stocks you can add to your portfolio today. Its decline of about 13% last month was nearly as bad as Teladoc's. A part of the reason for that is that the company's business has not been doing well during the pandemic; because Mastercard is dependent on the strength of the economy, COVID-19 has had a negative effect on its business.

In the company's third-quarter results, released Oct. 28, net revenue of $3.8 billion was down 14% and net income of $1.5 billion dropped by 28% from the same period last year. CEO Ajay Banga noted that the company is "seeing encouraging progress in the trajectory of domestic spending, while travel spending remains a challenge."

Over the long term, as the economy recovers and things get back to some sort of normal, Mastercard's growth is likely to continue. In 2019, sales of $16.9 billion were up 13% from the previous year; over the past five years, they're up more than 78%.

The company is still expanding its business, announcing on Oct. 27 that it would be expanding Instant Transfer in nine new European markets. The service works with PayPal to enable customers to transfer funds from the online service to their Mastercards. It's currently available in the U.S. and several countries around the world.

As Mastercard continues to introduce new products and services while also growing along with the economy, there's lots of potential for the credit card stock to generate strong returns for investors over the long term. And with a smaller market cap than Visa, Mastercard could see more upside than its rival.

Which stock is the best of the three?

These are some of the top growth stocks you can invest in today. Here's a look at how they're all doing this year compared to the S&P 500:

MSFT Chart

MSFT data by YCharts.

Only Mastercard's been underperforming the index, while Teladoc has been a star, more than doubling in value thus far. If you can afford to add all three to your portfolio, that could be a great way to help diversify and set yourself up for some great, safe returns over the long haul. But if you can't, the stock I'd go with today is Mastercard.

While Teladoc's doing well and telehealth visits are on the rise, the trend has yet to prove itself outside of the pandemic. And it's debatable how strong the demand for telehealth might be if people aren't concerned about leaving their homes.

Although Microsoft is showing good growth numbers, its $1.5 trillion valuation gives me some pause, given that there may not be a lot more room for the stock to rise in value. And also consider that even during the pandemic, when demand has been high for its products and services, the company's growth has been relatively modest at 12%.

That leaves Mastercard as the most attractive option. Credit card companies may not be doing well right now, but like financial stocks, they're likely to perform better during stronger economic times. It may not be a popular buy today, but Mastercard shows significant potential for the long term as consumer and business spending recovers.