Fruit, oranges, apples, bananas, grapes will all keep out of the fridge.
Mixed nuts will keep your protein, fibre and good fat levels up.
Oats oats and more oats. You can have oats for breakfast, and also take a tuppaware to school or work with oats in to make a hot snack at work. Make them with water, if you need the taste of milk keep a supply of protein power or powdered milk handy. Oats+water+protein/powdered milk + raisens = tasty & filling.
Buy small quantities of veg that you will eat before it goes mushy. Potatoss, sweet potatos, onions, tomatos, peppers, carrots, spinach and even lettuce can last for a few days in the kitchen cupboard.
Tuna and other tinned fish.
Baked beans, the food of gods sent down from mount olympus.
Tinned sweetcorn.
Brown pasta, keeps forever and only needs boiling water to cook, throw in the ingredients listed above and you got a meal.
Brown rice, as above.
Also, add snacks in between those 3 meals, you probably aint eating enough which is why the gym is killing you.