If I know the output voltages, input voltage, how is best to calculate the three resistors needed in Excel or similar?
Excel has a function for permutations, so if you care to run all of E variations (not to mention power ratings and tolerances of 0.1% to 5%) you are going to end up with a DATABASE larger than Earth.. There is a reason why the E-standard was designed as such, and to save your sanity - do not ever think about going parallel more than 2 resistors. Even then, the effort is not going to justify the cost.
I have nothing to back this up but I have gone insane before along these lines, and the first approximation of the math in bash script was as expected, it ran for days and gobbled up gigabytes of disk space for a loose result.
Do not go beyond two resistors paralleled - and go there only when in dire straights for an odd value. Even then, you are going to loose some precision, since the approximation is a logarithm, and when there IS a PERFECT match you SHOULD have a discrete value of E192 of the same.
Now, the question is if you need it in:
0.125W, 0.250W, 0.500W, 1W, 2W or higher.
0.01%, 0.2%, 0.5%, 1% precision or lower.
... You got the idea
Do not think about it, do not go there. The variations of all these man-made objects is larger than Earth itself.