Small update with big news. I'm writing it with big relief, at least so far. So after research I realised that despite how weak they are, their main problem was splayed leg, which looked exactly like on video here
Although our ducklings couldn't even hold their heads up and had swollen eyes from rubbing their heads on lining. So I decided to try whatever i can what could possibly help as looking at them didn't though they will make it at all... I gave them baby vitamins and water, sorted their legs up and put the middle one in sock as she would not lay still. She seemed to be really relaxed and cosy in that position. And we left them.
Because the only healthy one (my hubby called her Helen and she is his princess

) is not good around them, jumping on them and poking their eyes we decided we need to get her companion. So we are just back from 5 hours trip to the other side of the country with 3 5 days old runners ( yes, we went for just one

).
And I couldn't even imagine what we saw when we were back. 2 weak ducklings had their metamorphosis. They were very active, sitting in normal position and even standing up and slowly walking. In only few hours. I know it's still early to be exited but I'm so happy anyway. Don't see them struggle and suffer. I will still monitor their legs to see how they walk. Should I try fix it again if they are not totally right or just wait if niacin will help? The one which had the biggest problem (yellow) is still quite wobbly but she is not even 1,5 day old yet. Also need to set up bigger brooder tomorrow morning as its quite busy right now. Will still separate 2 smallest ones till they are 100% alright. So thank you all for your amazing help and will definitely put another update tomorrow ❤