A sinking tide lowers all boats, including Google, an analyst says.

In a research note, UBS Internet analyst Benjamin Schachter warned that a deteriorating advertising environment in 2009 would hurt the financial fortunes not only of Yahoo and eBay, whose businesses have struggled, but also bellwether Google.

In the third-quarter earnings preview, Mr. Schachter cut revenue and earnings per share estimates for all three companies. Price targets also fell, with Yahoo going from $28 to $20, eBay, from $28 to $18, and Google, from $700 to $525.

In afternoon trading, shares of eBay added 0.8 percent to $17.96, Yahoo skidded 6.8 percent to $12.57, and Google fell 4.4 percent to $364.20.

While Google “is relatively better positioned than the others,” Mr. Schachter predicted that third-quarter results would come in slightly below consensus forecasts in part because of a deteriorating advertising trend in Western Europe and particularly the United Kingdom.

Earlier this month, eBay warned that third-quarter revenue would come in at the low end of its forecast of $2.1 billion to $2.15 billion. The company, meanwhile, said that earnings per share would be above the previously announced range of $0.39 to $0.41.

Mr. Schachter (no relation to the reporter) said eBay is undergoing “an identity crisis” in which it wants to remain true to its auctioneer roots, but also wants to “compete in fixed price listings to spur growth.”

As for Yahoo, the UBS report said softness in advertising and execution concerns creates “downside risk” in the quarter. Likely outcomes, according to the report: Yahoo turns around its business based on strength in display advertising; new management takes over and sells off non-core assets and focuses on display, or Yahoo is acquired, with spurned suitor Microsoft still the most likely merger partner.