Nuuska, as I'm in a noisy public environment at the moment listening and comparing was a bit difficult.
Thank you for the link...I suspected what you meant by part C was a different melody.
It's part of the folk process...when in Canada I sat in on many sessions with the Quebecois folk society musicians...a lot of the old time fiddle tunes originated in Celtic countries and came over with the Irish potato famine orphans that were adopted by Quebecois farm families in the 1800's...the orphans were allowed to keep their Celtic names and their music...the tunes all had a Quebecois accent, but were recognizable as Celtic descent.
Lots of the old tunes/songs have as many as 5 or more parts...most people don't know all of them, just the popular versions.
It's possible that the linked 'original' version is not the complete as written but the version published in the past that the video used.
Brad's link shows that we use the same verse melody as written originally, and a different, likely easier to sing/play melody for the chorus...no 'third part', part C melody...mostly the same or similar words as the original...doubt you will find an English version of a C part.
While not understanding the Finnish words and not knowing whether you're starting with a chorus or the verse in the recording, guessing it's a verse, it sounds like you're using a variation on the original chorus melody for the verse, different from what we commonly use...the same 'jingle bells' chorus melody that we use, not the original as written, and, yes, you do use a third part that we do not use...at least not commonly...that was not in the original written version, at least in the video.
I don't read music so someone with an education and good research skills might be able to solve the riddle.
My suggestion is to go with the folk process flow...to accommodate all the guests, you could play the third part, part C, as an instrumental break when you're singing the English version, the Finnish guests can sing along to that part if they wish and know the words...the English can listen to that part and maybe learn some Finnish language...doesn't matter, they'll all be bouncing along to the tune in any language...it's a party.:very_drunk:
kudos for trying to accommodate everyone.:watermelon::watermelon::watermelon: