I'd imagine 3 trains is quite likely.
Linnanmäki does get approx. 1 million visitors a year, so there's a need for decent throughput.
2 trains would probably run most of the time, but having a third would be a good option.
If it's like Taron (which I expect it to be with 2 launches and a similar ride system) it would likely run 4 trains at max capacity, but comfortably run 3 with a steady throughput.