need have a begin transaction on different branch

Hello everybody I work in a company where we develop on 3 projects in same
time, the 3 projects are under subversion and are not in the same
repository.

My problem is when we commit modifications on 2 or 3 projects we can break
the build.

My questions are :
 - Can we lock an entire project with a command (svn lock work only on
file)?
 - Is lock is a good command to make a transactional commit between
repository?
 - If I use svn lock I'll have a problem because there is no recursive svn
lock on directory?
 - Could you describe the solution you adopted if you had the same problem?

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BADIN Guillaume
Appartement 405
2 promenade des sapeurs pompiers
94800 Villejuif