Hi,im thinking about starting to speedrun this game and i want to start with hoverboard category,i watched some videos and saw few runners at the start of the race haveing all 3 boost bars.How do they do that cuz for me bars are always empty.

You have to gain bars after pressing Quit Race, and they'll carry over. So for Kalebo III you want to spin around in the spot I did in my video (I think...) and then you'll have full bars once you restart

Thanks!