In addition, flights from these three cities to Riga on Friday have also been cancelled.
Pilots for the low-cost carrier in Germany, Sweden, Belgium and Ireland have all said they plan to strike on Friday, the AFP news agency reports.
Ryanair said it would have to axe around 400 out of 2,400 European flights scheduled for Friday, affecting some 55,000 passengers.
Germany will be worst hit with 250 flight cancellations.
Europe's second biggest airline has been grappling with staff unrest since it recognized trade unions for the first time in December 2017, in a bid to ward off widespread strikes over the Christmas period.
But unions say little progress has been made on their demands for better wages and fairer contracts despite months of talks.