Any Check Engine Light on? Any codes present? How many miles on the car?
If the converters are all legitimately bad and this is not a misdiagnosis the converters could have been ruined by coolant entering the exhaust (intake, head gasket leak, etc.) or by running excessively rich (very poor engine performance due to chronic engine ignition misses, leaking fuel pressure regulator, etc.).
If the mechanics suspect an underlying problem then what have they done to find ths problem?