How can I hide SP3 and get other SP2 updates?
Go to the update screen and chose "Custom Install"
After the scan has finished, beneath the SP3 option is the option to chose other updates (or words to that effect)
You can then chose which updates to except, or chose the option to "Don't show me this update in the future"
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After choosing the "Custom Install"
I found the last time "fairly recently" it has changed slightly and I had to chose something first before the actual list was accessible, but it was pretty straightforward and hopefully you should understand it.
edit
Just a thought.
SP3 should only increase your HDD by about 1GB and not as much as you have said.
Although I do usually run the disc clean-up utility soon afterwards.
But I always have a backup, just in case.