I'm not superstitious. My current fears have nothing to do with the media's recaps of the Black Monday market debacle, which took place exactly 20 years ago this week. The companies I'm fretting about didn't even exist during Wall Street's 1987 tumble.

Instead, I find myself staring at the earnings calendar, eyeing the three dot-com juggernauts poised to shed some light on their third-quarter performance. So much can go wrong for these companies, especially now that they've seen their shares inch higher in recent months.

Yahoo! It's Tuesday
Though many investors will turn toward Yahoo! (NASDAQ:YHOO) on Tuesday afternoon, no one expects a miracle. In fact, Wall Street predicts that earnings will fall considerably from last year's third-quarter showing.  (Our Foolish Forecast on Yahoo! sheds some light on what to expect tomorrow.)

Yahoo! would get lost in a crowd under normal circumstances. We've gone through way too many quarters of disappointing financials from the search giant, embarrassingly followed by breakout results from Google (NASDAQ:GOOG) two days later.

The difference this time will come from new CEO Jerry Yang, and what he says during the conference call about his self-imposed 100-day period of introspection. Over the past three months, Yang and his executives have been poring over the company for ways to improve it. Whether they plan to shed nonproductive properties, emphasize growth areas, or sharpen the company's cost structure, the weight is on Yang's shoulders.

I like many of the moves that Yahoo! has made since Yang took over. However, I fear that if Tuesday's announcements are anticlimactic, Wall Street will throw in the towel on Yahoo!. Its shares have surged 28% since bottoming out two months ago. Next month's Alibaba.com IPO is helping to fuel interest in Yahoo!, but investors will want something more organic to keep the good times rolling.

eBay Wednesday
Meanwhile, eBay  (NASDAQ:EBAY) faces questions of its own. Now that the company has all but admitted to overpaying for Skype, investors should be critical of the slowdown at the company's namesake auction marketplace business. The second quarter saw listings decline in the United States and Germany. Now eBay is slashing listing fees in an extended promotion in the United Kingdom, suggesting that things may be running sluggish across the Atlantic as well.

Should investors zero in on eBay's marketplace performance? The company has grown into a diversified company. PayPal is a true star, holding up well against a laundry list of Web-based financial-transaction enablers. Meanwhile shrewd acquisitions of companies like StubHub and Shopping.com haven't been saddled with Skype-esque accounting write-offs.

So why am I worried about eBay's report? Well, the company has beaten analyst estimates in each of the four past quarters. The stock climbed 33% this year. As with Yahoo!'s buoyant shares, those gains won't last if mortality rears its ugly head. With weakness at Skype and potential stagnancy in its auction business, eBay's got a lot on the line come Wednesday.

Googling for Thursday
Google reports used to be slam dunks. The search-engine star obliterated expectations in its first five quarters as a public company ... then stumbled on the sixth. It then went off to rattle another five market-thumping quarters ... and stumbled again on the 12th.

History would find Big G hopping back on its horse, trotting past the pros as it reports on Thursday. But what if 13 proves to be an unlucky number here?

It shouldn't happen, but Google is spending a ton of money on expansion lately. Some of it appears to be paying off. Deals with SINA (NASDAQ:SINA), Tianya, and China.com parent CDC (NASDAQ:CHINA) have helped the company gain market share in China. Rumors of a Google cell phone won't die. It Nokia (NYSE:NOK) or Research In Motion (NASDAQ:RIMM) might not be losing any sleep over such a possibilities, but Gphone chatter could surface in a conference call question on Thursday afternoon.

Expectations are so high that Google's stock blazed past the $600 mark with ease last week. One bad Thursday, spooking the market with back-to-back quarterly disappointments, will be a quick way to duck back below that mark.

Nervous trading days ahead
So now can you appreciate my concerns? Three online bellwethers have put on a lot of market-cap weight in recent weeks. They'll have to justify those gains over the next three days.

It's going to get messy. Even if the stocks don't spill, expect investors' gnawed-off fingernails to litter the floor like peanut shells at a roadhouse.

Cross your fingers, but bring a mop.

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.