Are you talking about custom themes or standard themes? When I upgraded from 2 to 3 all the themes were there. But by default only iDVD 3's themes are shown. If you go into the drop down box above the "list" of themes you should be able to selet "show all themes" or something like that and you'll see both iDVD 2 and 3 themes.
they are suppose to all be there, they are on every machine ive ever used idvd 3 on. make sure you turn on all themes, just not the new idvd 3 ones. thats probably what your encountering.