How can I show an unchangeable date that is 3 working days in the future (so not including weekends and/or bank holidays)

To clarify, you want to display the date 3 days from the current date, but that should exclude weekends or bank holidays?

Unfortunately, it is quite impossible to determine the bank holidays, unless of course you have all the dates listed in your end. Special holidays would also be a problem. We can exclude the weekends as they are fixed.

Do you have a form that we can check and an example case so that we can better replicate it in our end?

So far you can limit the dates your user can select. Here's a guide about it: https://www.jotform.com/help/337-How-to-Use-The-Limit-Dates-Option 

We'll wait for your response.