Apple referred indirectly to the three20 framework in their last Developer Conference and stated it is not necessary to use third party frameworks for photo viewer style scrolling applications. Three20 is fantastic coding, but is now outdated and solves a problem that has since been resolved by Apple's standard library code (e.g. photo scroller with zoom - Indeed Apple claim there was never a problem in the first place, but that there was insufficient documentation for developers to know how to do photo scrolling efficiently). Unless you want to implement the other specific three20 features you are best off avoiding using it. Check out the developer conference videos on advanced scroll views.
The most relevant video is from WWDC 2010 - assuming you have iOS dev center membership, you can check this, which is the most relevant:
and look for the session "Designing apps with Scroll Views"
Also check this one may be of interest:
and look for the video session "Advanced ScrollView Techniques"
Also check out the video on paged scrolling from WWDC 2012.