The 3.0 upgrade for previous versions of WSUS is being posted to the WSUS catalog today. So, once you complete your next sync, you should have it as an option.

From the WSUS Team Blog:

Just wanted to let you know - today is the day two updates will be available to you to assist in your management and deployment of WSUS 3.0.

First ,we are making an update available for WSUS 2.0 SP1, which will create a 3.0 "self update tree" on your WSUS 2.0 /SP1 servers. The self-update tree update contains the code for the WSUS 3.0 client. That means, once this update is approved and installed on your WSUS 2.0 SP1 server, 2.0 clients "mapped" to that server will be "self updated" to the 3.0 client the next time the client 'checks in' with the WSUS 2.0 SP1 server. The server remains WSUS 2.0 SP1, while the clients are able to update themselves from the server, to the 3.0 version. We did this to ensure customers had a distributable method to update their clients and realize new performance optimizations, but still manage the plan and timing to upgrade the servers to WSUS 3.0. This update is classified as a critical update for all server categories on which WSUS 2.0 SP1 is supported, with the exception of SBS R2.

Additionally...