Radio buttons need to be in a group together so only one can be selected. If they are in separate cells, it wont work.
However....you can just write some simple script so it works without it actually being in a group. You could do the same with checkboxes, rather than use radiobuttons.
Here is what i would do:
Basically, when one is clicked, the other two go off.

form1.#subform[0].Table1.Row1.a.rb.button::click - (JavaScript, client)
Table1.Row1.b.rb.button.rawValue = "0";
Table1.Row1.c.rb.button.rawValue = "0";
form1.#subform[0].Table1.Row1.b.rb.button::click - (JavaScript, client)
Table1.Row1.a.rb.button.rawValue = "0";
Table1.Row1.c.rb.button.rawValue = "0";
form1.#subform[0].Table1.Row1.c.rb.button::click - (JavaScript, client)
Table1.Row1.b.rb.button.rawValue = "0";
Table1.Row1.a.rb.button.rawValue = "0";