If you did this all the time it could affect your weight loss. The 1/3 speed rule is to limit the amount of free food you eat at a meal
In your meals case if you also had 1/3 of your plate filled with mushrooms and tomato for example it would mean that you would be eating less of the higher calorie free food to satisfy your appetite
One meal wont make much difference though so enjoy your egg, chips and beans!