The stock market's wild fluctuations over the past few weeks are reminding investors just how valuable safe, blue-chip stocks can be at a time like this. Having investments that you don't have to worry about can make your investment strategy that much easier: You just buy once and watch your portfolio grow over the years. Below are three investments that could be perfect for such an approach.

1. Thermo Fisher Scientific

Thermo Fisher Scientific (NYSE:TMO) makes a broad array of lab equipment and products that are used in labs and for diagnostics. Its broad diversification of revenue makes the stock a very stable long-term buy. In 2019, more than 41% of its revenue came from its laboratory products and services, 27% from life sciences, 22% from analytical instruments, and 15% from specialty diagnostics. Its top line also grew by 4.9% in 2019, showing steady increases in most of its segments.

In each of the past six years, Thermo Fisher's profit margin has also been well over 10%, and there's little reason for investors to worry that will change anytime soon. With good diversification and strong profit margins, Thermo Fisher's business looks very healthy.

Bar chart showing numbers getting higher.

Image source: Getty Images.

The company's also very versatile, as it's playing a big role in helping to contain the coronavirus pandemic. The Food and Drug Administration (FDA) approved Thermo Fisher's coronavirus test, which will help increase the number of people who can be tested for COVID-19. Thermo Fisher plans to produce 5 million tests every week as early as April.

While producing coronavirus tests may have a short-term effect on Thermo Fisher's financials, it's a great example of the company's strengths and ability to adapt to rapidly changing situations. And those are traits that investors should look for in long-term investments in order to feel comfortable that companies can overcome adversity and make the most of opportunities over the years.

Investors won't get rich from Thermo Fisher's dividend, which currently yields a modest 0.3%, well below the S&P 500 average of about 2%. But investors can enjoy stability and long-term gains. In the past year, shares of Thermo Fisher are down just 4%, even amidst this recent sell-off. That's all the more impressive when you consider the S&P 500 is down 9% during the same period.

2. Microsoft

Microsoft (NASDAQ:MSFT) is another big name that investors can rely on for safe, long-term performance. The trillion-dollar company has struggled in recent weeks, but it's still up 30% over the past 12 months. While the coronavirus may cause a problem for the company's employees, Microsoft's business will remain solid over the long term. As more people work from home, employees will continue to use Microsoft's Office 365 products -- including its cloud-based storage solution, OneDrive -- which will help companies stay connected.

Like Thermo Fisher, Microsoft is a versatile company, one that can roll with adversity. It doesn't shy away from social responsibility, either. It's recently partnered with the Centers for Disease Control and Prevention (CDC) to create a chatbot that will help people self-assess themselves for COVID-19 and whether they need to go to a hospital. There's a fear that there will be a shortage of hospital beds due to the fast-spreading coronavirus pandemic, and any way to lessen the need for beds will help hospitals weather the storm.

While the world faces instability, Microsoft remains a strong brand, and it's likely to stay that way for a long time. The tech company's profit margin was more than 30% in fiscal 2019. And in each of the past 10 years, at least 13% of the company's revenue has trickled down to the bottom line. Microsoft's a top brand in the tech industry. It was a great buy decades ago, and it will continue being a strong investment for the foreseeable future.

It also pays investors a dividend that today yields 1.4% per year. The company hiked its payouts in September, marking the 10th straight year it's done so.

3. Coca-Cola

Coca-Cola (NYSE:KO) is a favorite stock of Warren Buffett's, and it's easy to understand why. The company's popular Coke products remain a staple in many homes across the world. And even as tastes change and consumers look for healthier options, Coca-Cola has adapted. It has more than 500 brands around the world, including Dasani and Smartwater, two popular options for bottled water. Just like the other long-term buys on this list, Coca-Cola's been a versatile company.

Unfortunately, the bearishness in the markets of late has sent shares of Coca-Cola down, and the stock's fallen 7% over the past 12 months. But it's not as bad as the S&P 500's decline of 9%. And to help soften that blow a bit more, investors in Coke also earn a dividend that yields around 4% per year. That said, it's much more than a dividend stock. Coca-Cola is a Dividend King that's increased its payouts for 58 years in a row. It's one of the safest dividend stocks that investors can own today.

While COVID-19 may put a dent in the company's financials, there's room for Coca-Cola to absorb the hit; for the past two years, the company's profit margin was over 20%. And only once during the past 10 years has its profit margin been below 15%.

All three stocks can offer a good mix of diversification and dividends

Whether you're a value, growth, or dividend investor, the three stocks listed above offer a little bit of everything. If your focus is on growth, then Microsoft is likely the stock that you'll prefer. But if it's dividend income and long-term value, Coca-Cola is easy to justify, especially now that it's not far from its 52-week low. It's hard to go wrong buying shares of any of the stocks listed above, and they can be solid pillars to build your portfolio around for many years.