I'd suggest leaving Question 19 blank until you go in to claim, and then ask the officer who is dealing with your claim.
You are working 3 days, so you can only claim 2 days. Jobseeker's Benefit is normally calculated on a 6 day basis (i.e. Mon - Sat) but in cases where you are claiming because your working days have been reduced, the social welfare week is 5 days. So you will get 2/5 of the weekly amount.
You can't claim Jobseeker's Benefit if you are working 4 days a week. I suggest that you direct this question to the Officer dealing with your claim and they will advise you of how to proceed.