If I buy Adobe Photoshop Lightroom 3 as a student version can I upgrade it later?
I'm not 100% clear on what you mean by "upgrade it later".
There's no difference (other than price) between the Adobe's
"educational license" software and the retail versions.
You can't get Adobe's upgrade plan for software you buy at the
special educational price, but the software itself is supposed to
be identical to the full version... you don't need to upgrade it,
unless a new version comes out, and even then the educational price
is generally pretty similar to the cost to upgrade from the
previous version.