All of these are imprecise layman's terms that can be used to refer to a person's 'insides' , or, in fact any part of the abdomen. 'Tummy' is an informal word we use when talking to children. It's quite a sweet and friendly type of word. Adults also use it to sound gentle and informal. An interesting cultural difference between France and the UK is that whereas the French tend to medicalise conditions and refer to them in a serious and scientific way, the British like to trivialise and infantilise them. While a French person would say that they were suffering from a case of 'gastroentérite', a British person would say they had 'a bit of an upset tummy'. 'Belly' is also informal, and means the same as 'tummy', but it's less cute. It's a slightly uglier and more vulgar-sounding word. 'Tummy' is a nice word, 'belly' less so. Children have tummies: the fat old man you don't want to sit next to on the beach has a 'belly'. We often talk about 'beer bellies', with the same slight feeling of disgust. The only time that 'belly' sounds OK is in the term 'belly button', which is quite nice. 'Stomach' has two meanings. In some contexts, it can be a precise anatomical term, referring to the organ between the oesophagus to the small intestine. Or, in everyday contexts, can mean the same as 'tummy' or 'belly' i.e. the abdomen in general. When used in this sense, 'stomach' is less informal than 'tummy' or 'belly'. It is a neutral word that you could use in any context.

Stomach: technical and neutral term Belly: informal and also often coupled with "big" or "fat" to suggest a person is overweight or obese Tummy: a child's word for stomach or used by a parent asking a child "do you have a sore tummy?"; That's a very quick summary. Others may have other suggestions

The proper word is 'stomach'; 'belly' and 'tummy' are used colloquially, 'tummy' for speaking to children. Someone might say 'I've got a belly-ache' ; 'she's got a big belly' or 'he's got a beer belly' i.e. a big belly from excessive drinking