CD-ROMs are so 1988, and we forgot about ESX 2.5 upgrade tarballs decades ago. As such, you’ll want to grab the freshly-minted “Upgrade package from ESX Server 3.0.x to ESX Server 3.5,” unzip it on a web server somewhere, and upgrade all your ESX 3.0 Servers with:
esxupdate -r http://your.web.server.com/3.5.0-64607 update
That’s what all the cool kids are doing, anyhow.
(It also looks like a great way to fix those ESX Servers you built with the sorta-almost-released ESX 3.5 from a week ago. Not that anybody did that. Nope.)