Over the last 24 hours a "radio-controlled landmine" laid by the rebels blew up some 50 kilometres (30 miles) southwest of insurgent stronghold Lugansk killing the three servicemen, military spokesman Andriy Lysenko said.
Lysenko said that the situation was "most tense" on the western outskirts of the separatists' largest stronghold Donetsk, insisting that the "enemy regularly opens fire on our positions".
"In case of further escalation of conflict ... Ukrainian military commanders will have to return previously withdrawn weapons to the contact line," he warned.
President Petro Poroshenko on Sunday decried a sudden re-escalation in the conflict with pro-Russian separatists after a Ukrainian soldier was killed in clashes in the country's east.
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Violence intensified in recent days with six soldiers killed over the weekend despite a September truce agreement.
The fighting had dropped to almost its lowest level since the start of the conflict following the start of the latest in a line of Western-brokered truces.
Over 8,000 people have been killed and more than 18,000 injured in Ukrainian conflict since April 2014, according to the United Nations.
The observers said they had recorded an increased number of incidents involving mines and unexploded ordnances.