[Bamboo Cloud docs]
[Bamboo Knowledge Base]
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:
../Plan_key_for_A/
Using a set up like this, your library module (A) should only be checked out once across the Bamboo instance.