To get to Parc des Trois Frontieres in Belgium by public transport, you can follow these steps:

Find the closest train station to the park. The closest train station to the park is "Plombières" (in French) or "Blieberg" (in Dutch).

Check the train schedules to get to the closest station. You can use the Belgian Railways website to check the train schedules: https://www.belgiantrain.be/en.

Once you arrive at the station, take a bus to reach the park. From Plombières or Blieberg, take bus number 396 to reach the park. You can check the bus schedules on the public transport website of the region where the park is located: https://www.infotec.be/ (in French) or https://www.delijn.be/en/ (in Dutch).

Get off the bus at the stop "Drielandenpunt" which is the name of the stop for Parc des Trois Frontieres.

Note that the park is located at the border of Belgium, the Netherlands, and Germany, so it might be worth checking if there are any public transport options available from those countries as well.