Stora Enso, UPM-Kymmene and Metsliitto are each facing a fine of 1.02m (FIM 10m) after being found in breach of Finland's Act on Competition regulations.
The findings follow two years of investigation, during which the Competition Authority found indications of illegal collaboration aimed at improving the supply of wood in Finland.
These are said to have taken place during meetings held in the Mikkeli region.
"We had these meetings for better exchange of information about the wood market," said Stora Enso senior vice president Voitto Plkki.
"Inflated wood prices" led the authority to suspect the groups of participating in illegal purchase deals.
The matter was originally passed to the Competition Council in March, when it decided a 2.04m fine should be imposed on the three.
"We will consider our position before deciding whether to appeal," said Plkki.
The Competition Council deemed that the exchange of information "created a channel for collaboration between the companies".
If all three companies decide on an appeal, it will be lodged with the Supreme Administrative Court.
Story by Andy Scott.
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