It is more than 14%. It looks like this plant, all three reactors together, might eventually cover a third of Finland's needs. But those are 2020 numbers. Increased electrification, especially EVs and moves away from gas, will probably reduce that percentage. In that context, the third reactor is only incrementally more powerful than those already running at the facility.
"The Olkiluoto plant consists of two boiling water reactors (BWRs), each producing 890 MW of electricity, together comprising 22% of the country's electricity generation for 2020.[1] A third reactor, Unit 3, is expected to be online in January 2022, and at 1,600 MW, will by itself satisfy 14% of the country's electricity demand."
"The Olkiluoto plant consists of two boiling water reactors (BWRs), each producing 890 MW of electricity, together comprising 22% of the country's electricity generation for 2020.[1] A third reactor, Unit 3, is expected to be online in January 2022, and at 1,600 MW, will by itself satisfy 14% of the country's electricity demand."
https://en.wikipedia.org/wiki/Olkiluoto_Nuclear_Power_Plant