You can use standard FS relations for predecessors, and set task constraint type 'As late as possible':

1 milestone - 2,3 2 Activity A - 4 3 Activity B - 4 4 Activity C

You should set 'as late as possible' constraint on tasks 2-4, and set task 1 to the required fixed date ('start no earlier than' constraint).

You can use standard FS relations for predecessors, and set task constraint type 'As late as possible':

1 milestone - prerequisites: 2,3
2 Activity A - 4
3 Activity B - 4
4 Activity C

You should set 'as late as possible' constraint on tasks 2-4, and set task 1 to the required fixed date ('start no earlier than' constraint).