It has yet to happen, but some day a public company could reach a total market capitalization of a trillion dollars.

Market cap is simply the number of shares outstanding of a stock multiplied by its current share price. These days, many companies, such as Pfizer Inc. (NYSE: PFE ). JP Morgan Chase (NYSE: JPM ) and BP (NYSE: BP ). sport market caps in excess of $100 billion. Fewer still, are companies with market caps in the hundreds of billions — General Electric Co. (NYSE: GE ) and IBM (NYSE: IBM ) are good examples.

Even fewer have what it takes to continue growing at the rates needed to reach a trillion-dollar market cap. But I’ve come up with three companies that just might do it.

1. Petrobras (NYSE: PBR )

Business: Energy

Market capitalization: $191 billion

P/E ratio: 9

Back in December. I wrote that this Brazilian energy giant Petrobras had the potential to reach a total market capitalization of $1 trillion. The stock is up a modest 4% since then and still possesses what I think is huge potential for shareholder gains in the coming years.

For starters, its track record of value creation is there. In the past decade, average sales have been up about 17% annually, while annual earnings have advanced at an even higher rate, close to 23%. This has pushed sales up from less than $23 billion in 2010 to more than $140 billion in 2011. For the coming year, analysts expect another solid increase to $150 billion and earnings per share (EPS) of roughly $3.30, an incredible increase from only about $0.50 a decade ago.

Another decade of similar growth — and who knows — sales could approach $1 trillion. This is quite possible because Petrobras plans to spend more than $224 billion in the next five years to explore for oil and gas in presalt reservoirs off the Brazilian coast. The current price-to-sales multiple for this stock is 1.34, which, if sustained, would push Petrobras’ market cap well past the $1 trillion point.

2. Apple (Nasdaq: AAPL )

Business: Technology devices and applications

Market capitalization: $468 billion

Apple is another company that is proving to have one of the most powerful business models in the world. During the past decade, it has revolutionized the music industry (iPod, iTunes), the market for cellular phones (iPhone) and has virtually created the tablet device market with the introduction of the popular iPad.

The proof of its successes is in the numbers. Sales have advanced by 23% annually, or from less than $6 billion 10 years ago to well in excess of $100 billion today. Earnings per share have been up a stellar average of 43% in each of the past 10 years, from $0.05 to more than $30 last year. And growth appears to be accelerating — the past five years have seen sales jump 35.5% annually and earnings growth of close to 65%.

At this rate, Apple’s growth trajectory could push its market cap to $1 trillion in less than five years. This assumes the stock will advance 20% each year during this period. This is quite possible, because the underlying business fundamentals look extremely encouraging.

For starters, Apple’s current generation of products continues expanding across the globe. In addition, the company has its sights set on revolutionizing the media market with a TV device as well as the online media storage and retrieval market, through its iCloud service.

The current price-to-sales ratio is 3.7, suggesting sales need to reach $270 billion for the $1 trillion market cap. At a sales growth rate of 18%, this could occur in four years — just half of the sales growth rate Apple has achieved in the past five years.

3. Cisco (Nasdaq: CSCO )

Business: Network and communication devices

Market capitalization: $109 billion

Networking equipment provider Cisco was well on its way to being the first firm to reaching a $1 trillion market cap. This was more than 10 years ago, during the dot-com bubble.

Back then, the stock traded at more than 120 times earnings and would have broken the trillion-dollar barrier within a few years of the peaking of the bubble in early 2000. The bursting of the bubble clearly pushed this feat off by many years, but Cisco’s operations have steadily continued moving forward. It has benefited primarily from facilitating the movement of Internet traffic through its routers, switches and related communications devices.

The potential for Cisco to reach a $1 trillion market cap is mainly because of the expectation that Internet traffic will continue to expand. The company controls an estimated 60% of the current market and projects wireless data traveling through mobile networks will jump 18 times from current levels. Cisco sees 10 billion mobile devices in service by 2016, more than the entire Earth’s population (which should be around 7 billion by that time).

Current sales are only about $45 billion, meaning the $1 trillion market cap level could be more than a decade off. But if Cisco plays its cards right, then it could very well garner the lion’s share of this growth.

Risks to Consider: It’s not a given fact that these companies will reach that $1 trillion mark. There’s a reason it hasn’t been done yet: the kind of consistent growth needed to reach this level is incredibly difficult to achieve. Nevertheless, I think it’s possible.

Action to Take —> As I detailed above, I think Apple has the best chance at being the first to cross the $1 trillion market capitalization threshold. The simple fact that it’s half the way there makes reaching this level less of a hurdle. Additionally, there is little indication that its growth rate is slowing from its current double-digit annual pace of around 20%.

Petrobras also has highly-compelling growth prospects and is backed by the Brazilian government, a major shareholder that can help it fund the billions it will take to discover new oil and gas reserves. Cisco has a smaller base from which to grow, but the vast potential for the Internet to keep growing suggests it is highly likely to eventually reach the trillion-dollar market cap level.