Can multiple plans share a common 3rd-party directory
At this stage, Bamboo doesn't support having multiple checkout directories per build plan. However, you can work around this by setting these three directories up as separate Bamboo build plans - P A, P B and P C.
To make this work, you will also need to specify as an argument to your build scripts for P B and P C the location of A, which will be something like this:
Using a set up like this, your library module (A) should only be checked out once across the Bamboo instance.