Sales Via Flickr Elliott Brown

Source: Flickr user Elliott Brown.

Obviously, no one knows when the stock market or individual stocks will stop falling; but if you're bargain hunting investment ideas, and you're young enough to take a chance owning a few risky stocks in your portfolio for the long haul, it may be a good time to consider these three companies.

Todd Campbell: If you've still got a long way to go to retirement, you can take a bit more risk with your portfolio. In that case, Portola Pharmaceuticals (NASDAQ:PTLA) is one of my favorite ideas.

Portola Pharmaceuticals is a clinical-stage $1.86 billion market cap biotech that could have its first drug hit the market this year. The FDA is scheduled to decide whether or not to approve andexanet alfa in mid-August. If the drug gets approval, it will become the first therapy approved to reverse the effects of factor Xa anticoagulants.

That's potentially huge because factor Xa anticoagulants are multi-billion dollar per year therapies that are increasingly being prescribed to prevent blood clots. Because hospitals currently have limited treatment options when factor Xa patients are admitted due to a major bleed, or because they require emergency surgery, andexanet alfa could become a blockbuster.

Additionally, Portola Pharmaceuticals is using its factor Xa experience to craft its own factor Xa drug, and results from a phase 3 trial for the prevention of venous thromboembolism are expected soon. If results are solid, Portola Pharmaceuticals could launch its factor Xa drug next year, opening up another multibillion dollar market opportunity for the company.

Tim Green: Predicting what any technology company will look like 10 years out is tough. Technological advances will render some business models obsolete, and once-dominant companies can very quickly run into all sorts of trouble. When time is on your side, investing in technology stocks can be a great idea, as a mistake won't mean much when you have decades left before retirement. But when capital preservation is the most important thing, many tech stocks simply aren't suitable. Graphics chip designer NVIDIA (NASDAQ:NVDA) is a great example.

I own shares of NVIDIA, and I think the company has a bright future. NVIDIA is dominant in the GPU market, and it's aggressively targeting the automotive market, aiming to make its chips the center of the eventual self-driving car. Enterprise use of accelerators, such as NVIDIA's GPUs, is rising, as processing the massive amount of data being created by an increasing number of Internet-connected devices requires incredible computational horsepower. Facebook, for example, is using NVIDIA's Tesla GPUs in Big Sur, the company's AI and machine-learning platform.

Despite NVIDIA's long-term growth potential, the stock is risky simply because there are a lot of ways for things to go wrong. NVIDIA's automotive platforms may prove to be too costly, or a different approach may end up winning the battle for the self-driving car. In the enterprise, Intel is set to launch a new accelerator of its own, Knights Landing, which could threaten NVIDIA's grip on the accelerator market. And in the core GPU market, while the PC as a gaming platform has proven to be resilient, there's no guarantee that this will remain true forever. For retirees, tech companies without vast economic moats should probably be avoided.

Matt DiLallo: I'm a few decades away from retiring, and that's if my portfolio starts to cooperate. Still, with plenty of time on my side, I can take a bit more risk than a retiree can. One stock that fits that mold is SolarCity (NASDAQ:SCTY). While it appears that the company has a bright future, my investment could also get scorched if the thesis doesn't play out as planned.

That thesis is simple: With the globe growing more concerned about climate change, renewable power sources must increase. However, despite the significant drop in the cost of installing a rooftop solar system, not everyone who wants a solar system can afford one.

Enter SolarCity, which installs solar systems on the rooftops of consumers, and leases the systems to these customers under long-term contracts. It's a win-win: Consumers get clean energy while lowering their electricity costs, and SolarCity gets a long-term revenue stream.

The potential problem with this idea is that SolarCity is footing the upfront costs for these system installs, which takes a lot of capital. The company has borrowed heavily to fund this buildout, and will need to continue to borrow in the future, which carries its own set of risks. In addition to that, the company is focusing more of its attention on installing and managing larger projects for commercial customers, which could prove to be a big challenge.

Finally, the company is pretty dependent on both government incentives and climate change worries. If either were to go away, it could cool off the company's growth prospects.

Those risks aside, SolarCity has a massive opportunity in front of it, with the company estimating that its total addressable market in the U.S. is $60 billion and growing. That's the type of outsized opportunity that could help me to retire a bit earlier than I'd hoped -- if it works out. And if not, I still have plenty of time to make up for getting burned by a solar stock.

Matt DiLallo owns shares of Facebook and SolarCity. Timothy Green owns shares of Nvidia. Todd Campbell owns shares of Portola Pharmaceuticals. The Motley Fool owns shares of and recommends Facebook and SolarCity. The Motley Fool recommends Nvidia. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.