Harmful microbes multiply in a cereal porridge with nuts, fruits, veggies if you keep it in the fridge for 2-3 days?

Very slowly: Refrigeration at 35-38 F degrees, meaning not hitting the 40 F mark, lets microbes slow down and go sort of dormant. That means they either don't multiply or multiply very slowly. Porridge is a cooked, boiled food, and thus is essentially germ-free until people start touching it and a few germs start falling into it from the air. Properly-handled porridge should be fine for 3 days in the fridge.

BC: Bacillus cereus is a common organism to get a stomach infection from eating cereal. However, this is usually associated with eating the food after sitting at room temperature.