Is there a single formula (or series of formulas I can apply to one cell) that will allow me to determine the true expected principal payment from a loan portfolio with several loans of varying terms (all at different life cycles) varied (yet fixed) APRs.

I've tried this using a 3 loan model, running each through it's own amortization schedule, and then using a weighted average APR and term to compare. They don't add up. For example, 3 loans, each with an initial amount of $2000, an APR of 59.9%, and terms of 12,6, and 3 months, respectively, end up having a different first-period principal payment ($1054 in this case) as opposed to the weighted average model ($737).

I pray there is an equation that will allow me to see the correct expected principal without having to run each loan through an amort. table (or in this case, the CUMPRINC function in excel for period 1) given that I need to do this for thousands of loans and several portfolios!

Thanks!