We introduce a notion of depth three tower of three rings C ⊆ B ⊆ A with depth two ring extension A |B recovered when B = C. If A = EndBC and B |C is a Frobenius extension, this captures the notion of depth three for a Frobenius extension in [12, 13] such that if B |C is depth three, then A |C is depth two (a phenomenon of finite depth subfactors, see [20]). We provide a similar definition of f...