If you are thinking of the 3-country select pass for 5 days, that will cost you $385 for the adults and somewhat less for the youth. The problem is that the adult passes are only for first class, and the youth is 2d. Assuming you will ride together, the adults are paying 1st class prices to ride in 2d class.
You'll also have to pay a supplement from Interlaken to Grindelwald, but it won't be much. The 3-country pass won't help you much at all if you are thinking of riding the lifts and high mountain trains in Switzerland.
Even tho' it's late, I'd check and see if you can get a discounted fare from Stuttgart to Zürich. Then add up the fares in Switzerland (get them from the Swiss rail site, sbb.ch or rail.ch for English). Once you get to Grindelwald, the hops from there to Bern and Bern to Luzern are short, around 30-35 CHF. Normally, I wouldn't consider using a pass for those, but if the other legs of your journey are expensive, it might work out.
I checked on the rough cost of tickets for your Swiss travel (regular price, not special fares) to see if any kind of pass would help. The total is around 177-180 CHF, with 137 of that going to the inbound and outbound journeys. (Those two coincidentally add up to pretty near the cost of a Transfer Ticket, at $118, so that won't benefit you either).
You might be able to bring that down a little with "special offer" tickets for portions if it, but as Lee said they are probably no longer available.
You should re-check those fares yourself for your actual dates, but, if the total cost in Switzerland is around 180 CHF, I can't imagine the cost of your travel in Germany would ad enough to make it worth buying the passes.
