When the markets went over a cliff in March, there were plenty of bargains available for investors to add to their portfolios. And if you did buy stocks during that time, chances are you're enjoying some great returns today. Since hitting a low on March 23, the S&P 500 has risen a whopping 58%. 

But as strong as the index has been in recent months, Novavax (NASDAQ:NVAX)Tesla (NASDAQ:TSLA), and Square (NYSE:SQ) are scorching-hot in comparison. Over the same period, each is up well over 300%, with the best-performing of the three up nearly 800%. With such incredible returns over the past several months, let's take a look at whether these stocks are due to cool down and if it's too late to add them to your portfolio today.

1. Novavax

Starting with the best: Novavax shares are up more than 2,400% this year. If you invested $40,000 in this stock at the start of the year, you'd be a millionaire right now. The big reason for the rally is no mystery: It's the coronavirus vaccine. The Maryland-based company's vaccine candidate, NVX‑CoV2373, is in the latter half of its phase 1/2 clinical trial. The company released encouraging phase 1 results on Aug. 4, with the vaccine causing no severe adverse side effects.

Yellow question mark standing out among others.

Image source: Getty Images.

On Aug. 24, the company announced it was beginning enrollment for its phase 2 trials, and it hopes to get up to 1,500 volunteers. That's up from 131 people who took part in the phase 1 trial. 

Novavax is anticipating interim data from the results before the end of the year. A positive showing there could give the stock another boost. However, investors need to remember that good results from phase 1 are by no means a guarantee that it will succeed in phase 2. Experts say there's a 58.2% probability that a vaccine will make it from phase 2 to phase 3; the likelihood rises to 85.4% when moving from phase 3 to approval.

With Novavax reporting just $35.5 million in revenue and a loss of $17.5 million in its most recent quarterly results (released Aug. 10 for the period ending June 30), the company needs its vaccine to be successful in order to justify its sky-high valuation. It's currently trading at a price-to-sales (P/S) ratio of 85 -- well up from a multiple of about 5 just a year ago.

Unless you're a gambler, this is a healthcare stock you'll want to avoid, as there's too much riding on the vaccine. An underwhelming showing in phase 2 could spell disaster for Novavax's stock price.

2. Tesla

The electric car manufacturer has been doing well as of late, recording profits in four straight quarters. With profit margins of 2% or less during that time, those profits haven't been astounding, but they've been steps in the right direction for the Palo Alto, Calif.-based business. In 2019, Tesla incurred a net loss of $862 million, which was an improvement from the $976 million loss it reported a year earlier.

Elon Musk's company is doing well even amid the coronavirus pandemic. It released its second-quarter results on July 22 for the period ending June 30, and sales of $6 billion smashed analyst expectations of $5.4 billion. In the second quarter, Tesla delivered 90,650 vehicles, up from 88,400 in the previous quarter.

Tesla's shares are doing so well this year that the company announced a 5-for-1 stock split on Aug. 11. That's led to another surge in the stock's price. Today, it trades at a price-to-earnings (P/E) ratio of 333 based on its future earnings and a P/S multiple of 17. With so much hype surrounding this stock, investors may want to think twice about buying today.

3. Square

With contactless payments growing in popularity amid the pandemic, it's no surprise that San Francisco-based financial company Square -- which facilitates those payments -- is also doing well. That said, its return since the crash, at about 320%, is the most modest on this list. 

On Aug. 5, the company released its second-quarter results, also for the period ending June 30, and sales of $1.9 billion were up 64% year over year. They were also up 39% from Square's first-quarter sales of $1.4 billion. Square's mobile payment service, Cash App, has been critical to the company's growth. The service allows users to send and receive money, as well as convert cash into bitcoin.

In Q2, Cash App helped generate bitcoin revenue of $875.5 million -- seven times the $125.1 million the company earned from bitcoin for the same period last year. But despite the strong top-line numbers, Square still incurred a net loss of $11.5 million, up from a loss of $6.8 million in the prior-year period.

These trends toward digitized payments likely aren't likely to go away, certainly not as long as social distancing continues. But at a forward P/E of 278 and a P/S of 13, Square is another stock that looks awfully expensive right now. Even with future growth, it's going to take a lot for those multiples to come down. 

Should you consider buying any of these stocks?

Here's a quick glimpse of how these stocks have performed since the market crash:

^SPX Chart

^SPX data by YCharts

Those are some astronomical numbers, and it's hard not to be a little concerned that these stocks may be vulnerable if there's another correction in the markets. Unfortunately, all of these stocks are too expensive to be good buys today.

Even though they all have considerable growth potential, a lot of that future growth is already baked into their share prices today. And with no telling how long the coronavirus pandemic may last, it would be risky at best to buy such a stock based on a future that looks very uncertain.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.