It looks like nobody here were using Mainstage 3. So it is hard to give any advice for Mainstage 3 and GarageBand '11. Mainstage 2 used to install at least the instruments needed for the JamPacks in GarageBand's Instrument library as well as in Logic's, but not all instruments. Check, if you now have at least the JamPacks instruments in GarageBand. Set the pop up menu at the top of the Inspector to "Show all" and look, what else is now available besides the basic GarageBand collection.
You may need to rebuild the Loop index:
Since Mainstage 3 is advertized to work well with Logic, but GarageBand is not mentioned on the Apple page (or I missed it), the additional Jam Pack instruments may have been installed in Logic's library but not GarageBand's.
Compare the Instrument Libraries for Logic and GarageBand. But carefully, you will not want to change anything:
Look at the folders:
/Library/Application Support/GarageBand/Instrument Library/Sampler/Sampler Instruments/
/Library/Application Support/Logic/Sampler Instruments/