SBB on Tuesday confirmed news reports about the launch of the service, which was initially announced in February.
The free internet access will be limited to users for an hour a day, the state-owned rail operator said.
After an hour, access will be charged at the commercial rate, the ATS news agency reported.
Twenty other stations will offer the service by the end of the year.
The country’s 31 biggest stations are set to be equipped with Wi-Fi by mid-2014, while another 69 are to have the service by the end of 2015.
Tests are already under way at the three stations, which SBB is to reveal in September.
No plans are yet in place to provide free wifi access on Swiss trains because of technical barriers, ATS said.
In February SBB said it would be installing telecom repeaters in the railway carriages of trains on its major routes to improve reception for passengers using mobile devices, such as smartphones, tablets and computers.
The repeaters are to be installed between now and the end of 2014.
Switzerland’s three mobile operators (Swisscom, Sunrise and Orange) are also expanding their 3G and 4G/LTE telecom networks along rail routes.
This is expected to eliminate gaps in network coverage that passengers currently experience.