To group the radio buttons, but all of them must have the same name value. This will allows only one to be selected at a time and applies required to the whole group. Try this code to do the same:
<label><input type="radio" name="gender" value="male" required>Male</label>
<label><input type="radio" name="gender" value="female">Female</label>
<label><input type="radio" name="gender" value="other">Other</label>
Note: Setting is required for all the inputs, but not necessary (unless dynamically generating radio-buttons).
Note that to avoid the confusion as to whether a radio button group is required or not, authors are encouraged to specify the attribute on all the radio buttons in a group. In general, authors are encouraged to avoid having radio button groups that do not have any initially checked controls in the first place, as this is a state that the user cannot return to, and is therefore generally considered a poor user interface.