Investment company Elliott Management has offered to buy WAN optimization vendor Riverbed Technology for just over $3 billion while giving Riverbed a chance to entertain higher bids. Elliott started buying Riverbed stock last September and now owns about 10.5 percent of the company, according to an offer letter sent Wednesday that Riverbed filed with the U.S. Securities and Exchange Commission. Elliott said growth is slowing in Riverbed's core business and the company's efforts to diversify so far have hurt its value.