What is JotForm?
JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
At JotForm, we want to make sure that you’re getting the online form builder help that you need. Our friendly customer support team is available 24/7.
We believe that if one user has a question, there could be more users who may have the same question. This is why many of our support forum threads are public and available to be searched and viewed. If you’d like help immediately, feel free to search for a similar question, or submit your question or concern.
Unfortunately you can only have entire field as required or non required.
The only way to do this in the way you have mentioned would be to add textbox fields instead of full name field.
For example, adding 3 textboxes. One for first name, the second for middle name and the last for last name.
Then setting the first and last as required, while leaving the middle one as not required.
You would be able to use Shrink option to have the same fields in the same row.
You can see more about this one the following guide: How to Position Fields in JotForm