Time Warner Cable will buy Insight Communications Co. for $3 billion in cash as it bolsters its presence in the Midwest, the company said Monday. The privately held cable operator serves more than 750,000 customers in Indiana, Kentucky and Ohio. It has about 57,000 high-speed data subscribers, 679,000 video subscribers and 297,000 voice subscribers. Insight is owned by The Carlyle Group, Crestview Partners, MidOcean Partners, members of Insight management and others. Time Warner Cable Inc., based in New York, said that it expects the deal to create annual cost savings of about $100 million, with the majority of those savings coming within two years of the deal closing.