What is the difference between the words INCREDIBLE and UNBELIEVABLE and UNIMAGINABLE?
These adjectives are interchangeable and mean the same thing.
They all describe something which is so amazing that it cannot be imagined or believed.
I had an UNBELIEVABLE meal last night. (It was good beyond words.)
There adverbial forms are INCREDIBLE, UNBELIEVABLY and UNIMAGINABLY.
He plays tennis incredibly well. (His level is beyond words.)
You can substitute any of these adjectives or adverbs as you like:
My father left me an incredible amount of money.
My father left me an unbelievable amount of money.
My father left me an unimaginable amount of money.