Assuming you have a contract stating the fixed price and the three changes, you simply refer to that very contract.

Having delivered the contractually agreed upon work, I am happy to work on 
further refinements. However, this will require payment of invoices and 
agreeing on a future hourly rate.

Insist on an hourly rate, as this benefits the client as well as you.

Continuing in a fixed price mode most often leads to a souring of the client relationship, as they naturally try to extract as much additional work from you, often with no issue being too small.

Agreeing on a pay-as-you-go hourly rate means every change costs them money, which limits their nice-to-haves and will result in a more rapid final delivery and probably avoids ending badly.

Assuming you have a contract stating the fixed price and the three changes, you simply refer to that very contract.

Having delivered the contractually agreed upon work, I am happy to work on further refinements. However, this will require payment of invoices and agreeing on a future hourly rate.

Insist on an hourly rate, as this benefits the client as well as you.

Continuing in a fixed price mode most often leads to a souring of the client relationship, as they naturally try to extract as much additional work from you as possible, often with no issue being too small.

Agreeing on a pay-as-you-go hourly rate means every change costs them money, which limits their nice-to-haves and will result in a more rapid final delivery and probably avoids ending badly.