As general the required fields would be marked with an asterisk (*). Here comes the dilemma: There are some fields in the form which are required, but as a group(e.g 3.a/3.b/3.c). It's unnecessary to fill out all of them, is there any solution to express the meaning of 'at least one, but not limited?'.
Rather than presenting all three options at once, I'd suggest using a combination of a dropdown instead of a static label to choose which of the options (3a, 3b, or 3c) the user wishes to enter into the text input and, if the user is allowed to enter more than one option in the group, an 'add another' button that opens up another set of dropdown/text input fields for the user to add their second, third, fourth, etc.