Performance ranking by many managers and without functions

Dear All,
In my company we have a performance management system in which every manager rates his team.
The problem that we have a team which must be rated by many managers (3 managers). But we don't want that the employee's score be set by a function or a percentage (for example manager A influences the score by 40%, manager B by 30% and manager C by 30%, we don't want that kind of notation).
Have you please any idea how can we manage this ?
Thank you :)
Regards,

If the team had performed exceedingly well naturally there cannot be any apprehension about the PA rating even if the rating to be made by different Managers. So what made you to think of 40 > 30 >30 downward rating, any slippage some where? Without knowing who's who how do you expect us to suggest a way out to make it flat ? What can be done now is only to make up any shortfall or slippages and convert it 'a job well done'.