The investigation, which has been described as preliminary, follows complaints filed with the European Commission by a UK price comparison site called Foundem, a French legal search engine called ejustice.fr, and a German search site called Ciao that was recently acquired by Microsoft, Google said.
"While we will be providing feedback and additional information on these complaints, we are confident that our business operates in the interests of users and partners, as well as in line with European competition law," Google said.
The Commission, which is the EU's highest antitrust authority, often conducts a preliminary investigation when it receives complaints about potentially anticompetitive behaviour. It typically studies the market in question, interviews companies involved and then decides whether to launch a full investigation. Google appears to be the target of such a preliminary probe.
The search giant implied, but did not say outright, that Microsoft may have had a hand in two of the complaints.
"Regarding Ciao, they were a long-time AdSense partner of Google's, with whom we always had a good relationship," Google said in its blog post. "However, after Microsoft acquired Ciao in 2008 (renaming it Ciao from Bing) we started receiving complaints about our standard terms and conditions. They initially took their case to the German competition authority, but it now has been transferred to Brussels."
Foundem complained that Google's algorithm pushes the shopping site down in its rankings because it considers it a competitor, Google said. Foundem is a member of an organisation called ICOMP, Google said, "which is funded partly by Microsoft."
Microsoft could not immediately be reached for comment Tuesday evening.
The complaint from Ejustice.fr is based on similar grounds to the one from Foundem, Google said.
"Though each case raises slightly different issues, the question they ultimately pose is whether Google is doing anything to choke off competition or hurt our users and partners. This is not the case," Google said in its posting.
"We always try to listen carefully if someone has a real concern and we work hard to put our users' interests first and to compete fair and square in the market. We believe our business practices reflect those commitments."
The Wall Street Journal first reported the probe earlier Tuesday, calling it a preliminary investigation that is at the "early, fact-finding stage."