The cost per day for the first 5 days with a select saver railpass, assuming you use all five days, is $83.60/day, per person.
Additional days up to 10, the incremental cost is:
Day 6, $46.00/day
Days 7-8, $43.00/day
Days 9-10, $40.50/day
Days 11-15, $58.50/day (last 3 intervals assume you use all days of the interval).
At today's exchange rate, you three days of travel, PER PERSON are:
Frankfurt-Stuttgart, $34.30
Stuttgart-Munich, $20.30
Munich-Vienna, $86.20
It certainly looks like the first two days, Frankfurt-Stuttgart-Munich, are not worth buying two extra days of the pass. Of course, if it's the difference between a 10 day and a 15 day pass, and days 11-13 are really expensive, it might not be worth leaving off Frankfurt to Munich. But I suspect you will have other "light" days in there too.
The only way you can be sure is to consider all days. You can get Austrian prices from www.oebb.at and Italian prices at www.trenitalia.it. What other countries are you going to? Remember that travel by Italian Eurostar with a railpass carries a BIG supplement; your railpass only covers part of the cost.
When you get a cost for each leg, order them from highest to lowest and compare the cost for the day to the incremental cost of an extra day of the railpass. I'm sure you will find the last, least expensive days to be far less than the incremental cost of the railpass. Keep whittling away days until you find the number of days, if any, for which the railpass pays.