The combination would marry Time Warner's trove of programming and Google's popular search and e-mail services with Comcast's high speed Internet portal and experience in cable video distribution and telecommunications.
Time Warner and Comcast declined comment. A Google spokesman declined comment on the talks, adding, "Google and AOL have a healthy global partnership and AOL remains a valued partner."
One source warned that the discussions over a joint investment were still in its early stages and could fall apart.
AOL contacted Comcast earlier this summer and has held separate discussions with Google over potential investments, the sources said.
Although Google's discussions over its interest in AOL's free Web portal were progressing, Google had contacted Comcast last week over possible interests in executing a joint investment, one source said.
Since then Google and Comcast have discussed the possibility making a joint minority investment in the online unit of Time Warner Inc. (TWX).
Google and Comcast have not yet drafted a formal plan to invest in AOL, the sources said. One source said the deal, albeit attractive to Comcast, was not "critical" to the company.