I understand how the tranches are created and why they are created. Few things I want to ask:
Which tranche has principal only? Which has interest+principal? Which has only interest? Why is tranche 3 the longest maturity tranch? Why is tranche 1 the shortest maturity tranch? Which tranche has the highest prepayment risk?
CMOs can be chopped up many ways; don’t confuse one tranche scheme with another.
If the CMO is chopped into a PO tranche and an IO tranche, then, well, the PO is principal only, and the IO is interest only.
If the CMO is chopped into sequential tranches, then the A tranche gets paid off first, then the B tranche, then the C tranche. A has the highest contraction risk (risk of getting paid off sooner than expected); C has the highest extension risk (risk of getting paid off later than expected).
There are lots of other ways a CMO can be chopped up. (I analyzed these for six years at PIMCO.) These two ways are arguably the easiest.
And the tranche A in your case can have PO or IO or a mizture? So, what differentiates the returns of all these three tranches? The way I see it is that CMO prepayment risk is same as mortgage passthrough risk. Just that I banks divided them in a fancy way to cater to diffr people. How would they possibly allocate principle or interest to these tranches?
In a sequential CMO, Tranche A gets principal and interest until it’s paid off. Tranche B gets interest only until A is paid off, whereupon B starts to get principal. Tranche C gets interest only until B is paid off, whereupos C starts to get principal.