I'm not sure why you are using code for this. Doesn't a rule, like I suggested in the PM I sent you, work?
If you want to use code, you'll need to get the repeating node and loop through it, setting the value of the field.
For example (C#, 2007 Object Model):
XPathNodeIterator nodes = MainDataSource.CreateNavigator().Select("/my:myFields/my:group1/my:group2", this.NamespaceManager);
foreach (XPathNavigator node in nodes)
{
node.SelectSingleNode("my:field3", this.NamespaceManager).SetValue("yes");
}