Three major types of basic fringe benefits for employes -- pensions, group life insurance and health insurance -- cost their bosses an average of 12.3 percent of payroll annually, according to a survey of more than 400 companies conducted by William M. Mercer Inc., a benefit consulting firm.

The Mercer survey covered mainly large firms and showed that the costs for the three fringe benefits was highest in the utility industry, 13.8 percent, and lowest in retail, 5.7 percent. In all industries, pension were the biggest single fringe, accounting for half to three-quarters of the total.

By geographic area, the fringe benefit cost was highest in the Northeast, 11.2 percent of payroll, and lowest in the South Central states, 8.7 percent.

Although the average cost of pensions, group health and group life was 12.3 percent, 75 percent of those responding to the survey said they paid under 15 percent of payroll.

Mercer said that in addition to pensions, group health and group life, employers in most firms must provide several other forms of cash and noncash compensation beyond weekly wages and salaries.

These include: Social Security payments made by the employer, which add about 6 percent more to the 12.3 percent of payroll for the three benefits surveyed; workmen's compensation insurance; unemployment insurance; holidays; sick leaves; paid vacations and bonuses; and profit-sharing.