@Sviatogor Wow this is awesome! This legend just described fight between Perun and Veles. Old Slavic myth incorporated in legend. Even number three have strong mythological symbolism in old Slavic faith. In even older Indoeuropean myth you have great snake or dragon who stoped the water or rain. After fight with thunder and sky god, the snake-dragon is defeated and the water-rain is relased.
Thiskind of legend you have here in Croatia. Few km from me in small place Žrnovnica you have several toponyms which are connected with Slavic mythos. Above Žrnovnica is hill with name Perunovo, and beneth village in field there is a toponym named zmij kamik, snake stone. And of course there is a river near. Legend is similar with Belarussian…
Perun and Veles are know to all Slavs and they are most important two gods. We all benefit them from times of Protoslavic time. The time when all Slavs were under one “banner”