Hi,
I am Magento1 certified developer with "Developer Plus" certification having 8years of working experience in web development, along with almost 2+year of working experience in Magento2 web development, majorly in custom development, theme implementation and migration and site from scratch.
I have gone through your requirement, but requirement is not very clear, you like to add three fields to existing custom shipping method, but its not mentioned how those fields be used and where to display and how.
Lets discuss further in brief on your requirement so can give you proper estimation.
Thanks