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.
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Hello, to better understand what you're trying to ask, I checked some of your forms and found this http://www.jotform.us/form/62384517753159
I'm assuming you really are trying to limit the Radio Button selection. This is not applicable to Radio Buttons though since this field only accepts one selection and no more. It will only accept 1 option selected regardless if you use Jotform or if you're building a form from scratch using HTML.
A Checkbox on the other hand is the field that allows multiple selection. If what you aim is to limit only 3 options to be selected on a checkbox with 4 options, then that's doable with JotForm.
All you have to do is remove those Radio Buttons you entered on your test form > change them into checkboxes > then set the Entry Limits to your as needed
In case I misunderstood your question, please ensure you will give us more info on your next reply so we can better assist you.