In these expressions, ところ is used to indicate time. V dic.ところです indicates a point in time just before the action is started. Vているところです means that the action is in the midst of taking place. Vたところです indicates that the action has just been finished.
わたしはこれから食事をするところです。
I'm just about to have a meal.
わたしは今、食事をしているところです。
I'm having my meal now.
わたしは今、食事をしたところです。
I have just finished my meal.