16 days holiday per year is the correct statutory minimum. (i.e. 4 x no. of days worked each week).
So to work out your holiday entitlement for 3 weeks, instead of 52, you need to multiply 16 by 3 and divide by 52. This means that you're entitled to receive 0.923 of a day's holiday (or equivalent pay). If you've not already had any holiday, you'll be entitled to receive holiday pay for 5.54 hours.
However, if you've already had a day's holiday (e.g. if you normally work on a Monday and the firm was closed on 1st January), you'll have had slightly more time off than you're entitled to and the company would have the right to deduct 0.077 of a day's pay (equivalent to 0.46 of an hour) from your final week's pay.
Chris