You can accomplish what you're trying to do by using the common base class Control
:
IEnumerable<Control> controls = this.Controls
.Cast<Control>()
.Where(c => c is RadioButton || c is CheckBox || (c is TextBox && c.Name == "txtFoo"));
foreach (Control control in controls)
{
if (control is CheckBox)
// Do checkbox stuff
else if (control is RadioButton)
// DO radiobutton stuff
else if (control is TextBox)
// Do textbox stuff
}