You can check your (train) connection from Geneva airport to Berne [u]here
At this short notice there seems to be no time to buy a pass online in advance. Please check this [u]here
[u]Opening times of services
[/u] at this railway station can be found on mentioned link.
So I don't think you can purchase any pass at that time of the day/night.
I guess you have to buy a regular ticket to Berne at a ticket machine (accepting Swiss Francs, Euros and credit cards).
Buying any pass can be done later at any larger SBB railway station during opening hours. To give you an idea of some prices:
Geneva – Berne
Single ticket CHF 51
Berne – Interlaken Ost
Single ticket CHF 26
Interlaken Ost – Geneva Airport
Single ticket CHF 67
It's difficult to give you other prices if you are not sure yet what your plans are during these three days. I refer to the [u]Railway Faq
and the [u]online travel planner
[/u] so that you can see the prices yourself.
You will find ideas for day trips [u]on this page
[/u] or tips for hiking trips [u]here
The price of a [u]Swiss[/u][u] Pass[/u]
is CHF 260, for four days, second class.
As you may have to buy a regular ticket from Geneva airport to Berne and as you are not sure yet which trips you are going to make during your stay the Swiss Pass is probably not the most economical choice.
I suggest that you consider buying a [u]Swiss Half Fare Card
The price of this Swiss Half Fare Card is CHF 99, also second class.
Both Swiss Pass and Swiss Half Fare Card will give you discounts (with the Swiss Pass you can travel for free in almost every sort of public transportation – train, boat, bus).
Since you have no specific plans yet, I think it's better you overview the possibilities on the links given.
I hope this will help you and I wish you a very pleasant time there.