The price the booking system shows initially is a “from” price, so it is telling you that prices start at 3,50. But then when you enter your details it will show you the actual price that applies in your case.
The avail yourself of the cheaper 3,50 price you would need to be in posession of the “Half Fare Card” which is a discount card most Swiss have, and which you as a tourist can buy to if you want. But wether that is worth it in your case depends on what else you are planning.
Note that local tickets are only valid a few hours, so I would not buy those in advance. Certainly not when arriving by plane.