Microsoft Project does not work the way you expect it to do. If you do not know when tasks B and C will start, how would the software know? I would recommend you schedule these two tasks with a Finish-to-Start dependency between them, which would remove the resource overallocation. On the FS dependency between tasks A and B, you might want to enter a Lag time value of 10d or something like that to show that task B does not start immediately when task A is completed. Hope this helps.