With the stock market in turmoil, investors willing to stomach the volatility have access to some of the lowest prices in years. Well-known billionaire investors certainly aren't sitting on the sidelines. Instead, they're buying up shares of attractive companies at a discount, fighting the urge to panic that befalls so many individual investors. A few of our Foolish contributors have identified three stocks that billionaires are piling into. Here are the details.

Sean Williams: When we're talking billionaire investors, we're often talking about the most influential of all billionaires, Warren Buffett. One stock Buffett has been gobbling up of late as if it's going out of style is midstream and downstream oil and gas giant Phillips 66 (PSX -0.66%). It's also a company that billionaire Dan Loeb at Third Point has been fancying.

What makes Phillips 66 so attractive in the current oil environment is that every facet of its business can potentially benefit. The more than one dozen refineries that Phillips 66 owns are capable of processing 2.2 million barrels of oil per day -- and that's a good thing, because reduced prices at the pump are likely going to fuel gasoline demand (pardon the bad pun). When oil prices fall, crack spreads for refiners often widen, allowing them to take advantage of juicy margins for many quarters to come.

Secondly, we have Phillips 66's chemicals business, which also benefits when natural gas and other commodities drop in price. Lower input costs allow for margin expansion, which in turn leads to higher profits for its chemicals operations.

Even Phillips 66's midstream operations can benefit. With many shale companies buried up to their necks in debt, idling wells may not be an option. Even with prices as low as they are, producing and storing may be their only move. With pipeline and storage deals often conducted years in advance, Phillips 66's midstream operations tend to produce very predictable cash flow regardless of what wholesale oil and natural gas prices are doing.

Of course, there's also the company's superior 3% yield, which appears to be safe, with Wall Street projecting roughly $6.70 in EPS in 2016. If you're looking for a smart energy play with oil prices still weak, it might be worth following the money and checking out Phillips 66.

Tim Green: Eddie Lampert, CEO of Sears Holdings (SHLDQ), holds a major stake in struggling retailer Sears through his hedge fund, ESL Investments. But the billionaire has also been buying shares of Sears Hometown and Outlet Stores (SHOS), which was spun off from Sears in 2012. Following purchases made earlier this year, Lampert now owns, directly and indirectly, 50.7% of Sears Hometown and Outlet, up from 46.2% in April of last year.

Like Sears Holdings, Sears Hometown and Outlet has been struggling. Over the past 12 months, the company posted a $169 million net loss on $2.36 billion of revenue, although the company's revenue declines haven't been nearly as severe compared with Sears Holdings. Sales at Sears Hometown and Outlet slumped just 2.7% in 2014, compared with a 13.8% decline for Sears Holdings.

Sears Hometown stores are generally located in smaller markets and mostly owned and operated by independent retailers. Because of their location, Hometown stores face less direct competition than the stores Sears Holdings operates, although online retail may be threatening that benefit. Based on the numbers, Sears Holdings appears unlikely to survive in the long run, but Sears Hometown and Outlet has a better chance, despite the challenges it currently faces. It's unclear exactly why Lampert is loading up on Sears Hometown and Outlet, but it is undoubtedly a more attractive company than Sears Holdings.

Todd Campbell: Billionaire healthcare entrepreneur and former Teva Pharmaceutical Chairman Phillip Frost continues to build a truly massive stake in his company, Opko Health (OPK -1.60%).

Since December, Frost has purchased, either directly, or indirectly via his Frost-Gamma Investments, more than 47 million shares in Opko Health, and that's brought his stake in the company to a whopping 182.2 million shares.

Why is Frost, who left Teva Pharmaceutical to run Opko Health last year, so bullish? Well, there's a lot going on at Opko Health that could turn it into a major healthcare player. The company has orchestrated a dump truck full of deals to get its hands on intriguing drugs, such as Varubi, an anti-nausea drug approved last year that it licensed out to Tesaro (NASDAQ: TSRO), and Rayaldee, a vitamin D prohormone that the FDA is set to make a decision on in March.

Opko Health also made a big splash last year, when it bought specialty lab company Bio-Reference Labs. That deal gave Opko Health access to payers that it can use to drive demand for its 4Kscore prostate cancer test, and it also gave the company $800 million in fiscal '14 revenue to boot.

Overall, if Varubi succeeds, Rayaldee nets a go-ahead, and the Bio-Reference deal pans out, then following Frost into Opko Health could be profit-friendly.