SBC Communications will offer cable customers three months of free high-speed Internet and satellite television if they switch to SBC’s services, the company said Wednesday. The promotion, which SBC says could be worth up to $340 per customer, is the most aggressive attempt to date by a large local telephone company to lure customers away from cable. SBC’s move comes a few weeks after it lowered the price of its high-speed Internet service to $14.95 per month for new subscribers in its 13-state region.