Negotiators for 12,000 Teamsters Union drivers and warehousemen and eight major Southern California supermarket chains reached agreement on a new three-year contract and Teamster members ratified the agreement, according to representatives of both sides. Michael Riley, president of Joint Council 42 of the Teamsters, said the new pact provides a 50-cent-an-hour wage increase in the first year, 40-cent hourly hikes in each of the next two years and increases in pensions and health benefits. David Willauer, a spokesman for the Food Employers Council, said his organization would resume bargaining this week with unions representing thousands of Southland meat cutters. He said he also expected that those negotiations would be resolved amicably. Three years ago, the Teamsters and the meat cutters struck the markets for eight weeks.