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

db.collection.aggregate([{"$addFields":{"attrToSearch":{$function:{body:"function(mostImportant, lessImportant, notImportant) { return mostImportant || lessImportant || notImportant; }",args:["$mostImportant","$lessImportant","$notImportant"],lang:"js"}}}},{"$match":{attrToSearch:{$regex:"wer",$options:"i"}}}])