If I find myself constantly re-tying my laces because I don't think they are tight enough, then I replace my laces. It seems like they 'stretch out' over time and don't hold the tightness anymore. I've never worn a pair of laces to the point that I have visible worn spots but some skaters do!
Because the laces do tend to stretch out, I'd go with the smaller laces. I had to do that once for similar reason (I didn't want to have super-long laces) and although I could not lace to the top hook for the first week or so, once I'd worn them for awhile, the laces loosened up enough to get the top hook and eventually they became 'perfect'.
I don't know how much slack you have with your proper length laces and if you've used them for awhile, you may not remember when you first got them how much excess lace you had. 3cm is not that much, especially when you consider that means each end of the lace will be 1.5cm short (1.5cm on each side x 2) so it could be that those will work out for you.
If you're going to change out your laces before the competition, I'd give yourself at least 3-4 skate sessions to get used to them. I like to have bright white laces for competition so I tend to remove mine and wash them or swap out for new laces beforehand, and it's funny how doing that takes some getting used to. That could be a mental thing for me, though!
Good luck with your competition - hope you have a great time!