Stingy, miserly and frugal: What is the difference in usage?

First of all, frugal is not "mostly about food."

Second, the differences between the words should be apparent from dictionary definitions, but in case they're not:

Stingy and miserly are both pejorative. Frugal is usually used in a complimentary sense when applied to a person, and is neutral when describing an object (such as a "frugal meal").

Stingy is less severe than miserly. To call someone miserly is to make an extreme judgement about that person.

Nearly all of the words can be used interchangeably for the gist of the intended meaning.

However, there's a slight implication difference with regard to whether the intent is to be not generous, or not wanting to spend more than one has to.

A stingy person is likely not to be considered to spend money to do something "nice" or "go along with the crowd" in a purchase (generosity):

He won't contribute to the employee gift exchange? He's really stingy.

A miserly person spends as little money as possible (minimal expenditures):

The miserly old man lives off the road in a house and rarely entertains guests.

A frugal person will likely buy exactly what he needs but not more (exactly or minimally sufficient):

When he goes to the grocery store, he keeps to his grocery list, being frugal to only buy the items on it and only those for which a deal or coupons exist.

I would suggest (although this is entirely from my own personal idea of these words rather than from any particularly reputable source) that @robusto is correct.

However, while stingy and miserly are both pejorative, they are of different levels. For example, I would only describe someone as miserly if they were actively unkind as a result of their distaste for spending money. For instance, a person running a small business who would rather lose an employee and have to find a new one every year, rather than give the current one a small pay rise.

Stingy, however, I would use only to describe someone who only ever pays for the exact amount they use, or exact amount they've eaten. Or to describe someone who say, has lent you 20p when you're at the shop, and insists on having it back. In this example they aren't being in any way unfair or unkind, but it can be infuriating between friends.

Hope this clarifies the usage with some level of context - although I fear I might have made it more complicated!