Is your portfolio too heavily exposed to one sector? If so, you could be putting yourself at significant risk if there's a market crash. As investors in airline and travel stocks will tell you, the crash that took place in March didn't hit all industries equally.

Every downturn is different; the industries hit hardest during the financial crisis that took place more than 10 years ago may not be the same ones that suffer during a crash caused by a global pandemic today. That's why the safest thing to do is to diversify and to balance out your exposure. If you've got $5,000 available to invest, buying shares of the three companies below can help strengthen your portfolio.

1. Thermo Fisher

Thermo Fisher (NYSE:TMO) is a top healthcare stock that can add versatility, dividends, and overall stability to your portfolio. The Massachusetts-based company has been able to adapt to the COVID-19 pandemic by quickly obtaining emergency use authorization (EUA) for its Taqpath COVID-19 Combo Kit, which tests for SARS-CoV-2 (the virus that causes COVID-19), in March, when the pandemic wasn't yet the global crisis that it is today. The U.S. Food and Drug Administration has expanded the EUA multiple times since then.

Rook standing among fallen pawns.

Image source: Getty Images.

The company is also working with WuXi Diagnostics and the Mayo Clinic on an antibody test that will help to determine whether someone may have potential immunity against the coronavirus. 

By being able to adapt and pivot to the current pandemic fairly quickly, Thermo Fisher ably demonstrates its versatility. And that's something long-term investors will value.

It's paid off for the company thus far, with second-quarter results July 22 showing sales up 10% year over year. Management stated on its earnings call that revenue from COVID-19 tailwinds totaled $1.3 billion (total sales for the quarter were $6.9 billion).

Another benefit of owning shares of Thermo Fisher is its dividend. Although its yield is a modest 0.2%, well below the S&P 500 average of 2%, it's still a way to generate some recurring income every three months for your portfolio. 

2. Target

Target (NYSE:TGT) is a stable retail stock that's also doing well during the pandemic and would look great in any portfolio. The Minnesota-based company released its first-quarter results on May 20 for the period ending May 2, and comparable-store sales were up an impressive 10.8% from the prior-year period. The company attributes that to a 12.5% increase in the average size of a customer's basket of purchases, as consumers were in pantry-loading mode during the early stages of the pandemic.

One of the benefits of a big-box retailer like Target is that consumers can do all of their essential, day-to-day shopping at one of its stores. That makes Target much more resilient than a niche store, as it can serve customers who are in need of a wide range of products.

Investing in Target is a great way to add some retail exposure to your portfolio without taking on significant risk. In each of its past 10 quarterly results, Target's never failed to stay in the black.

Target also provides investors with a solid dividend, which today yields 2.2% per year. On June 11, the company announced it would be raising its dividend payments by 3%. It's the 49th annual raise in a row, meaning the stock is now one more annual hike away from becoming a Dividend King.

3. Twitter

Twitter (NYSE:TWTR) can help round out your portfolio with another key industry -- tech. The social media company is coming off a strong second quarter in which it reported 186 million monetizable daily active users (defined by Twitter as "users who log in and access Twitter on any given day through twitter.com or our Twitter applications that are able to show ads") on its platform during the period. That's up 34% from the prior-year period and 12% from the first quarter.

Although sales were down 19% year over year as advertisers cut back on spending, the increase in use is a great sign that there's still significant demand during the pandemic. And that could help bring advertisers back.

Twitter is the riskiest stock on this list, especially with President Trump looking to regulate social media companies and take away some of the legal protections they currently have. The company is also coming off a security breach where many high-profile accounts were compromised.

But these could prove to be short-term concerns for investors; they may, in fact, amount to nothing. The social media platform remains immensely popular, and whatever Trump's intentions toward Twitter, even he continues to use it. 

Twitter is the only stock on this list that doesn't pay a dividend, but it makes up for that with potential long-term growth.

Which stock is the best buy today?

Year to date, only Target has underperformed the S&P 500:

TMO Chart

TMO data by YCharts

To strengthen your portfolio, you're better off investing in all three stocks to diversify your holdings. However, if you only need to add one stock, then go with Thermo Fisher. It has consistently reported a profit over the past 10 years, and it's in the fairly stable healthcare industry, where demand will likely be a lot more consistent than in the retail or tech sectors.