Military spokesman Andriy Lysenko said fighting had intensified compared to a month ago and accused rebels of "actively using heavy weapons" including a Grad rocket launcher.
"Over the past 24 hours, as a result of the hostilities, three Ukrainian soldiers have been killed and eight more wounded", he said.
The spokesman said the fighting took place around the eastern rebel hub of Donetsk and the government-controlled town of Mariupol.
There has been an uptick in violence in the east of Ukraine, with seven troops reported dead on Tuesday and five more over the weekend.
Ukrainian Defence Minister Stepan Poltorak said last month it could take years to end the conflict, which has claimed more than 9,300 lives since it erupted in April 2014.
Earlier this month, France and Germany held fresh talks with Kiev and Moscow as part of efforts to try to seek a lasting peace deal but no consensus was reached over elections in the separatist regions of eastern Ukraine.