Paris to Berner Oberland for just 3 days? We need suggestions for our limited time in Switzerland.

Not a 2 country rail pass. Buy NOW tickets from Paris to Basel. You can get the 12:23 train for 100 euro total for 4 people. You will not find a better price. It is a nonrefundable, unchangeable ticket.
www.capitainetrain.com
In Switzerland, you could buy a 3 day Swiss Pass for 210 chf each. Its the most expensive way to go, but I leave it to you if it is worth it. It will get you from Basel to the BO (suggest Muerren) and discounts of 50 % on the lifts. and back to Basel Friday. The ticket cost is about 79 chf each way Basel to Muerren.
If you want to go back to Basel Saturday morning, you can also look at the Swiss Transfer Ticket with the Half Fare Card adder. That will also get you rt Basel-Muerren and 50% discount on the lifts and rails in between. It costs 201 chf and must be purchased BEFORE you arrive in Switzerland.
http://www.sbb.ch/en/leisure-holidays/holidays--short-breaks-in-switzerland/swisstravelsystem/swiss-travel-pass.html

"but don't know which city to travel to, where to stay, or how to organize our time."

For a home base in the Berner Oberland, I'd suggest staying either in Lauterbrunnen or Mürren. Of the two, Lauterbrunnen is more central to both sides of the valley and also the rail hub. You'll first need to travel to Interlaken Ost, and transfer to the small Berner Oberland train for the short 20 minute trip to Lauterbrunnen.

How to organize your time will depend on what you want to see in that area. I'd suggest having a look at the Switzerland guidebook to get information on sights that might interest you. Some of the usual favourites in that area are the Cable Car ride to the Schilthorn (used in the filming of the James Bond movie On Her Majesty's Secret Service), and the rail trip to the Jungfraujoch (highest rail station in Europe). Both trips are a bit pricey but worth seeing at least once if your budget will allow.

This is most helpful. Thanks for taking the time to share your wisdom.
I am still wondering about the rail pass. As I looked at rail pass options to get from Paris to Switzerland, and travel within Switzerland, it seemed simpler to just get the pass and then not worry about it further. With four of us traveling together, a small discount there also. But I am intrigued about just buying individual tickets.
Any additional thoughts to consider before making a decision?

Also, since we are four "seasoned citizens" all about 70, but healthy and active, wondering about good trails to consider once we get to mountains towns, and the best way to get into the mountains.

I truly appreciate your experience and information. Thanks.

Do you have other train journeys in France? If not, I would just buy the tickets point 2 point to Basel. This is a special deal and when the few tickets offered at this price are sold, they are gone. The "Lyria Senior" tickets would cost you 276 euro, so there are 176 euro saving by buying now on-line. If you try to travel on a Eurail pass, you might get shut out of the train you want because the number of pass holders allowed on the train may have been reached. Even if there are plenty of open seats left on the train, you will have to buy a ticket to get on or wait for another train that has seats available for passholders. In Switzerland, beyond Interlaken, you only get a 25% discount of lifts and mountain railways.
The convenience of a Eurail Pass comes at a cost.
Edit_ Itinerary would be Paris to Basel to Interlaken Ost to Lauterbrunnen to Muerren via a cable car and one more short train ride.

Chris, What a beautiful place -- the helicopter views of the Alps are stunning and beautiful.
Sam, Thanks so much for your advice. Very helpful for sure. Thanks for taking the time to share your knowledge.

I will point you back to Sam's original advice about reserving the Paris-Basel TGV now for the best price, then using a Swiss Travel Pass for hop-on convenience on most trains, buses, trams, boats, and museums within Switzerland. The 2nd-class version for 3 consecutive days costs about $225 per person when purchased in the US. There are several reasons why this is better than a 2-country pass, including better coverage in Switzerland and a TGV seat reservation for pass users that is higher than you'd pay to buy tickets now.

Thank you Laura. You have re-affirmed the good advice, all of which is helping us to be more confident that we are making a good decision -- and this will be greatly beneficial to our travels in June.
I appreciate your taking the time to offer your advice.