Add a sub in every activity you want to close,
B4X:
sub closeMe
activity.finish
end sub
Add a sub closeAll in a service module, such as starter
B4X:
sub closeAll
callsubdelay(C, "closeMe")
callsubdelay(B, "closeMe")
callsubdelay(A, "closeMe")
end sub
You can call
starter.closeAll in C activity after jobdone