If you are trying to do this in Javascript. You can use the following code :
var rblSelectType = document.getElementById('hdnRbtnList').value;
txtSelectedItem = document.getElementById('hdntxtTextbox').value;
var rbtnOne = rblSelectType + '_0';
var rbtnTwo = rblSelectType + '_1';
var rbtnThree = rblSelectType + '_3';
if (document.getElementById(rbtnOne).checked) {
txtSelectedItem.value = "Your Value";
}
else if (document.getElementById(rbtnTwo).checked) {
txtSelectedItem.value = "Your Value";
}
else if (document.getElementById(rbtnThree).checked) {
txtSelectedItem.value = "Your Value";
}
hdnRbtnList - will be your RadiobuttonList ClientID
hdntxtTextbox - will be your TextBox ClientID
Hope this helps.