CS5 upgrade from CS3

you can update your cs3 programs to the latest cs3 versions, http://www.adobe.com/downloads/updates/

but they cannot be updated to cs4 or beyond and owning cs5 has no relevance to what you can do to update cs3.

on the other hand, you may be able to use your cs5 upgrade IF, when it was purchased, your cs3 was a qualifying product. 

you could find that out by starting your cs5 install.  you will be prompted for your cs5 serial number.  after entering it you will be prompted for a qualifying upgrade product.  if you can select your cs3 product, you will then be prompted for the cs3 serial number and installation of cs5 should proceed.