I downloaded 3.3 DP1 and the splash screen says 3.3 SP2, so those are apparently the same thing...
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These are all complete installers. Device Pack 1 is newer than CP3 so it includes everything from CP3. The naming convention has changed over time. The Release Notes include a description of changes are in each release.
The reason 3.1 appears after 3.3 on the archive page is because it got dropped off the web by mistake and then added later.
So in summary, if you want to work on 3.3, download the latest offering of 3.3 irrespective of what the pack is. In this case, For 3.3, DP1 seems to be the latest.