NameValueCollection and using autopostback

[Nicole_new C# Programmer]

I am creating an application where I have 3 listboxes. The 1st holds the
attributes of our products like division, group, etc. The 2nd listbox holds
the values attach to each attribute like division may have the code ED or DC
and group by have ORP or DVS. Now I am suppose to allow user to select one
attribute and list the values associated to each attribute in listbox two.
Then once they selected the values they want to display this information in
listbox 3 like this for example "division->[ED, DC]". Now I need to
determine how to reflect a value has been selected previously in listbox 2.
Is it possible. I thought that if I could store all the attriubutes in a
NameValueCollection as keys and add the values selected in listbox 2 as the
values in the NameValueCollection.

In all I would check what values were already selected for a attribute using
attriubute as the key and parse out the values and if they exist in the
NameValueCollection highlighted those values somehow in listbox2. But since
I have listbox1 autopostback, my NameValueCollection seems to become empty by
the time I want to use it.

Any suggestions, what can I do to store what has already been selected???