It doesn't depend on bottle size, 3% is 3%
It does matter what mg your nic base is of course, if you put 5ml of 100mg nic in a 30ml bottle it wouldn't be 6mg nic it would be 16.7mg nic.
But 3% of pure nicotine (1000mg) would be 30mg. I assumed in saying 3% they meant 3% when I guess they meant 3% of 100mg which would be 3mg or 0.3%.
When talking about DIY I just automatically assume people are being scientific since nicotine levels are super important. I shouldn't assume anything and simply ask.