forms – Does displaying fields only after selection reduce conversion?

I’m designing a checkout flow, and it needs to ask the user for registration fields at 3 different times (this is necessary by the rule of the third party we are using). I know that this many fields is bad for the experience, so I thought of hiding the last field section at first, so that it is displayed only after the user selects the offer that will be available to him. The question is whether this is good practice or should I show the fields as soon as the screen loads.