Database
[{"_id":"64e03c733","A":"valueA","B":"valueA","C":"valueA"},{"_id":"64e03c734","A":"valueA","B":"valueB","C":"valueC"},{"_id":"64e03c735","A":"valueA","B":"valueA","C":"valueB"}]
Query
db.collection.find({$expr:{$not:{$or:[{$eq:["$A","$B"]},{$eq:["$A","$C"]},{$eq:["$B","$C"]}]}}})