My forking is a bit rusty, so hopefully, I'm getting this right. Since process A is the parent of process B, there should be a wait there. Otherwise if process A finishes, it will wait on process C only. So, if process C finishes before process B, you'll get a zombie. The following may work a bit better.

if ($pid = fork()) {
    if ($pid2 = fork()) {
        <process A>;
        waitpid $pid2, 0;
    } else {
        <process B>;
        exit;      
    }
    waitpid $pid, 0;
} else {
       <process C>
       exit;
}
[download]