[{a:1,"mostImportant":"qwer","lessImportant":"rty"},{a:2,"mostImportant":"","lessImportant":"weRt","notImportant":"asd",},{a:3,"mostImportant":"qwe2","notImportant":"asd",}]

db.collection.aggregate([{$addFields:{mostImportant:{$cond:{if:{$eq:["$mostImportant",""]},then:null,else:"$mostImportant"}},lessImportant:{$cond:{if:{$eq:["$lessImportant",""]},then:null,else:"$lessImportant"}},notImportant:{$cond:{if:{$eq:["$notImportant",""]},then:null,else:"$notImportant"}}}},{"$addFields":{"attrToSearch":{$ifNull:["$mostImportant","$lessImportant","$notImportant"]}}},{"$match":{attrToSearch:{$regex:"wer",$options:"i"}}}])