2 Sandwiches and a Yogurt seems pretty natural - it's about what I eat at work (convenience store).
If you are still feeling hungry, try (as others have said) eating brown bread. And drink water - you say you drink a lot, the recommended (by UK nutritionists anyway) is 2L of water a day (I'm always hungry as I don't drink enough). Too much water can cause a distended belly and various medical problems (as you loose osmosive pressure between cells and blood and so all the salts/sugars in your cells leave, causing massive damage to organs), but this is quite rare (recently it was a woman in the states who entered a drinking competition and downed 8L [or somethin] of water and died).
What kind of yoghurt are you eating? If it's a manufactured/flavoured one that may have more chemicals in it and thus not be so good for you (corn syrup as mentioned, though I think that's only the States). Try eating plain, natural bio-yoghurt (fairly low calorie), but add in some fresh blueberries or something to add a little flavour (and the goodness of the fruit too).
Finally, if none of that works, see a doctor to get your basal metabolic rate calculated. From that you can work out how many calories your body is using a day, and then you can work out how many to eat a day (either the same as your rate, keeping your weight roughly constant [depending on fat levels of foods]; eating at least 15% less calories for weight loss [can leave you feeling hungry but the body adapts over time]; or eating more [only to gain weight, and must be done in conjunction with exercise so it's not turned into fat]).