[{"_id":1,username:"abc"},{"_id":2,firstname:"abc"},{"_id":3,lastname:"abc"},{"_id":4,lastname:"ggg"}]

db.collection.find({$or:[{username:{"$regex":".*abc","$options":"i"}},{firstname:{"$regex":".*abc","$options":"i"}},{lastname:{"$regex":".*abc","$options":"i"}}]})