I am having lunch——我正在吃午饭or我在吃午饭。我现在吃午饭 is not appropriate. It''s like in English——I now am having lunch.
There is no difference between“正在” and “在” when you want to say“ you are doing sth.” If you want to express it with 现在,it should be like this——我现在“在”吃午饭。