coding triple integrals in VB6

I am trying to write a program that will calculate a triple integral. If the limits of integration are all constants, this is simply done with three For loops to calculate the Riemann sums of each and then multiply the three results together. My problem comes in trying to use variables in the limits of integration. I need to essentially take a string value and plug it into an equation.

This is related to a thread I started earlier but I was hoping somehow it might be a different enough situation that there might be a simple way of doing it. Now that I am writing it out it seems to be that the only answer involves complicated parsing, but I might as well throw it out and see if anyone has any ideas.

Thanks - Eric