Dynamic picklist

Is it possible to do the following (I haven't figured out a way yet):

Form A has 3 picklist fields (1, 2, & 3). I want the field3 picklist to show the values that were selected in picklist fields 1 & 2.

I tried the "Clear Picklist" code example listed in the Deluge Help section. I added the code as an "on update" when fields 1 & 2 were updated. But it resulted in only the field2 value appearing in the picklist. The Field1 value was overwritten once the script was run for field2. Any help is appreciated. Thanks.