According to church tradition Saint Andrew the apostle passed by the Black Sea. it was the beginning of Kyievian tradition however the conversion of the Slavs officially started with Saint Helen of Kyiv (before baptism she was known as Olga/Olha).
As for your question "I am aware this is the symbol of the sea god Poseidon of Greek mythology"
The Greek missionaries (priests) when the Pope sent them to Ukraine and Russia ¹ they used the Trident of Poseidon to explain the concept of the Holy Trinity since in that they had a similarity ²
This is how God gave the gift of Faith to Ukraine thanks to Saint Helen of Kyiv (before she was baptized she was called Olha of Kiev)³ and her nephew Saint Vladimir Svyatoslavich (Ukranian pronounced as Volodymyr) who converted the slavs ⁴
Similar to how saint Patrick in Ireland he used the shamrock to explain the symbol of the Holy Trinity
If you would like to know more you can also ask a Ukrainian Greek Catholic priest (they are fully of orthodox spirituality and liturgy only in communion with Saint Peter in Rome)
There is also a book that talks about it briefly called
"Church History" - Fr. John Laux, M.A.
When you combine both knowledge east and west rites history of Christian Catholic orthodoxy then it makes sense. This is why I say it is best you ask a Ukrainian Greek Catholic priest since they know better this part of history.
I am not a Ukrainian nor Russian however I love both cultures and traditions (communism/socialism Soviet Union is not a tradition nor culture it is something else that oppressed both nations, cultures , and it tried to first eradicate religion and then later it tried to disintegrate it but could not since our Lady protected those that were loyal to her son and her and the church, it tried to eradicate other cultures it oppressed but that is an entirely different matter and subject)
Going back to your question Ukraine and Russia have beautiful cultures history heritage and spirituality.
Tryzub is unique to Ukraine but it is also essential for Slavic cultures particularly in Ukraine and Russia the Greeks helped them understand that there is one single God in three person not three gods.
Similar as in the way Saint Patrick in Ireland explained there is three leaves in one single shamrock and it is the same plant same source of life.
Conversion of the Slavs, Sts Cyrillic and Methodius the Apostles of the Slavs, St. Olga and Vladimir the Great convert the Russians .” Church History A Complete History of the Catholic Church to the Present Day" by Fr. John Laux, TAN BOOKS AND PUBLISHERS, INC, 1945, pp. 280–285.
Zhukovsky, Arkadii. Trident, Internet Encyclopedia of Ukraine, 1993, www.encyclopediaofukraine.com/display.asp?linkpath=pages%5CT%5CR%5CTrident.htm.
I personally have not read "Poiasnennia tryzuba" But since the author cited it I am passing that source along:
2. Pasternak, Onufrii. Poiasnennia tryzuba, herba Velykoho Kyïvs’koho Kniazia Volodymyra Sviatoho (Uzhhorod 1934; repr, Kyiv 1991)"
- Zhdan, Mykhailo, and Arkadii Zhukovsky. Olha, Princess, Internet Encyclopedia of Ukraine
Additional source to support # 3.
Pritsak, O. ‘When and Where Was Ol’ga Baptized?’ HUS, 9 (1985)
4.Lencyk, Wasyl. “11 Facts about Day of Christianization of Kievan Rus.” Ukraine Celebrates the Day of Christianization of Kievan Rus, Internet Encyclopedia of Ukraine, 112.international/politics/ukraine-is-celebrating-the-day-of-christianization-of-kievan-rus-7716.html.
These other sources below I have not read it but i will still post it since it was within the article
4. Nazarko, I. Sviatyi Volodymyr Velykyi, Volodar i Khrystytel’ Rusy-Ukraïny (960–1015) (Rome 1954)
- Polons’ka-Vasylenko, N. Istoriia Ukraïny, 1 (Munich 1972)
that being said you do not have to believe me you can talk to any Ukrainian-Greek Catholic priest ,or Russian Greek Catholic priest and he will help you clarify and even correct my mistakes.
I am not Ukrainian Greek, nor Russian Greek
However I am very fascinated by their respective cultures and heritages and admire both countries and peoples. I am not saying nor implying is same culture nor same country. each one is it's culture and heritage yet spiritually Ukrainian spirituality is very important to Russia because Ukraine as you can see through this source God used them as the fathers that taught Russia the faith God used the Greeks to bring the faith to the Slavs (saint Andrew came to preach the Greeks as well).
That is not a myth that is a fact. there is many similarities because of Christian customs yet each one has their respective culture and sovereignty.
spirituality they are connected one to another but a priest can explain the history way much better than I can.