Two men and one woman were killed by sea mines while swimming in Odesa, according to Ukrainian media.
A local official confirmed the three had been killed by "explosive devices", at beaches close to Zatoka, where recreational swimming is banned.
The Black Sea has long been a popular holiday destination in Ukraine, but many of its beaches have been deemed unsafe since Russia's full scale invasion.
Officials have urged holiday goers not to swim in prohibited waters.
Witnesses told local outlet Dumskaya that the explosions happened at 11:30 (09:30 BST) on Sunday between Karolino-Buhaz and Zatoka.
