Radio button/s with out default selection?

Hello

I have placed 3 radio buttons set/group on my selection screen, say, rb1 rb2 and rb3.

As we knew by default the rb1 will be selected.

But, I don't want to have a default selection, hence all the 3 RBs should be open (not selected) so that user will be forced to select one rb.

Because I need to write a routine the moment user picks a RB

Pls. let me know how can I make RBs with out default selection?

Thank you