If you have $5,000 available to invest in stocks, there are some great options out there for you. The three stocks I have listed below are underrated investments and are trading at low earnings multiples -- and 2021 could be a great year for all three of them. You don't want to make the mistake of overlooking these potential bargains.

Buying shares of Jazz Pharmaceuticals (JAZZ -0.94%), Suncor Energy (SU 0.87%), and Western Digital (WDC 1.78%) can set you up for some great growth opportunities while also diversifying your portfolio with exposure to pharmaceuticals, cannabis, oil and gas, and tech. And while they are cheap buys today, they may not stay that way for long.

A group of business people talking outside.

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1. Jazz Pharmaceuticals

A year ago, Jazz was a slow-growing business that may not have attracted much interest from growth investors. Its annual revenue in 2020 totaled $2.4 billion, up a modest 9% from the previous year. And the year before that, its top line grew by 14%. Those were good numbers -- but not great. However, with the company's acquisition of GW Pharmaceuticals for $7.2 billion earlier this year, its prospects look a whole lot more promising.

GW Pharmaceuticals is known for its cannabis-based drug, Epidiolex. In its year-end results for 2020, the company reported sales of more than $527 million, up a blistering 69% from the previous year. And Epidiolex makes up nearly all of that revenue. The business is rolling out the drug to new markets in Europe, and the U.S. Food and Drug Administration (FDA) approved it for a new indication last year, to treat tuberous sclerosis complex (a hereditary condition that can cause seizures). Previously, the FDA had approved it for Lennox-Gastaut syndrome and Dravet syndrome -- two rare forms of epilepsy.

Now that Epidiolex is part of a large and profitable company, one that has considerable assets including cash of more than $2 billion, there will be an opportunity for its growth to accelerate as more resources are poured into it. Jazz trades at a forward price-to-earnings (P/E) multiple of just 13, which is cheap given that the average stock in the Health Care Select Sector SPDR Fund trades at 27 times earnings. If you're looking for a growth stock without an obscene price tag, Jazz is a solid option.

2. Suncor

Energy company Suncor is coming off a difficult year. COVID-19 shattered oil prices last year (even turning them negative at one point), with demand for any kind of travel kept to minimal levels. But better times are ahead for the industry, and some analysts are expecting the price of oil to soar to $100 a barrel by next year. That's an extremely bullish forecast -- the last time oil prices were at those levels was 2014.

The industry has been struggling since then due to an excess supply of oil. Companies have had to either get leaner, shut down, or amalgamate. Suncor has chosen to get leaner, and one example of that has been the deployment of autonomous haul trucks in an effort to bring down costs. While some investors may be spooked by the company's eye-popping loss totaling 4.3 billion Canadian dollars last year, that was largely due to impairment and other non-operating charges that totaled CA$2.2 billion. What's arguably more important is that the business is generating cash -- over the trailing 12 months, Suncor has brought in CA$227 million in free cash flow.

With oil prices improving and West Texas Intermediate, a key benchmark, now at over $70 a barrel (a year ago it was at about $40), now may be an opportune time to load up on a top oil and gas stock like Suncor. It's trading at a forward P/E of just 12 -- well below industry giant ExxonMobil, which is at a multiple of 18.

3. Western Digital

Western Digital is trading at 18 times its future earnings, but in the tech sector, that's cheap; the average stock in the Technology Select Sector SPDR Fund is trading at more than 32 times its profits. Although the company isn't posting scorching-hot sales numbers, that doesn't mean this year won't be a good one for the business.

When the company reported its latest results on April 29, sales for the three-month period ending April 2 totaled $4.1 billion, down 1% year over year. However, Western Digital is expecting stronger numbers to finish the year, projecting fourth-quarter sales to come in between $4.4 billion and $4.6 billion -- up from the $4.3 billion it reported in Q4 2020.

The main area that remains a challenge for Western Digital is in its data center devices and solutions segment, where sales were down 19% in the third quarter (its other segments were in positive territory). However, it did note that one of its newer products "experienced significant sequential growth" at what the company considers a "cloud titan." And as companies adapt to the new normal now that the economy is opening back up, the need for flexible storage options from Western Digital will likely be on the rise -- especially if more employees will be working remotely. While sales may not be great right now, that could change later in the year.

Although Western Digital isn't forecasting terribly high growth numbers in Q4, I wouldn't overlook this stock as it could be one that benefits from a strong recovery this year. Multiple analysts have set price targets of at least $90 for the stock, and that means buying it today could generate returns of close to 30%.