When we talk about future plans in English, the construction “to be willing to do” can be used with a few different adjectives (in place of willing), such as ready, prepared and able. In some contexts these can be used interchangeably with almost no difference in meaning, roughly meaning to be prepared to do something. The idiomatic expression ready, willing and able to … means to be incredibly eager / prepared to do something. It makes use of these multiple words reinforcing the idea of readiness/eagerness by repeating adjectives that with almost the same meaning (similar to a construction such as I am well and good). However the use of such a phrase does demonstrate there is some difference between these words, and in certain contexts the different forms of to be willing to do can have distinct meanings.

The different meanings of ready, willing, and able

In a neutral form, these three adjectives each say that you are prepared to do something, but from a slightly different perspective.

willing – prepared to do something as a choice, demonstrating eagerness
ready – prepared to do something in either a physical or mental state, demonstrating either physical or mental capacity or eagerness
able – prepared to do something as a possibility, demonstrating capacity to do it, not eagerness

In some contexts, ready and willing can demonstrate a meaning so similar that the difference is not noteworthy, such as in different forms of the sentence “I am willing to meet with you.” Although we could analyse a different meaning in the following three sentences, the understood meaning would be the same: I am prepared to meet with you.

I am willing to meet with you.
I am ready to meet with you.
I am able to meet with you.

The differences only become clear if there is a context that makes the meanings distinct.

I do not think you will get the job, but I am willing to meet with you because you have good references. (i.e. I have chosen to meet with you.)
I have finished interviewing the other candidates, so I am ready to meet with you now. (i.e. I am now capable of meeting you.)
I am able to meet with you today at 4pm, when I am free. (i.e. It is possible to meet you.)

The idea of possibility makes ready and able very similar here, although ready suggests more preparedness while able is simpler possibility. Note, though, that even in these contexts there is flexibility; to say “I am ready to meet with you because of your good references” would be understood the same way as willing.

When the differences become more distinct

In another context, the meanings would be quite clearly different. This is usually when we are talking about a task where the type of preparedness could be interpreted differently, for example when taking a test.

I am willing to take this test. (I have chosen to do so.)
I am ready to take this test. (I have prepared for it.)
I am able to take this test. (I have the ability to take it. / It is possible.)

The nature of preparedness may also be important if we are talking about a physical plan or action, where willing shows a choice/eagerness and ready or able shows possibility:

I am willing to go to lunch now because the queue will get very long later.
I am ready to lunch now as I have finished writing my report.

Willing is often used in this sense to show a choice that may be surprising or otherwise noteworthy – it emphasises that the action is being done as a result of a choice. This is the key difference, as ready and able emphasise that the action results from more basic preparedness or possibility. Again, in some contexts this emphasis does not make a difference, in other contexts it the meaning is important. It is to be especially emphatic, though, that we combine all three in the idiom, ready, willing and able: