digraph G {
compound=true;

node[shape=box];

subgraph cluster0 {
  color=blue;
  label="Top";
  a;
}

subgraph cluster1 {
  color=magenta;
  label="Middle";
  b;
}

subgraph cluster2 {
  color=green;
  label="Bottom";
  c;
}

a -> b[dir=back,ltail=cluster2,lhead=cluster1]
b -> c[dir=back,ltail=cluster1,lhead=cluster0]

}