Database
[{"fieldOne":10,"fieldTwo":20,"fieldThree":30},{"fieldOne":10,"fieldTwo":20,"fieldThree":30},{"fieldOne":10,"fieldTwo":20,}]
Query
db.collection.update({},[{$addFields:{fieldThree:{$ifNull:["$fieldThree",0]}}},{$set:{newField:{$add:["$fieldOne","$fieldTwo","$fieldThree"]}}},],{multi:true})